Charms are a fairly well contained card descriptor in magic. There is only one card that is "Something Charm" that does not adhere to the Charm formula and it proceeded all the Charms so we can forgive Wizards for Freyalise's Charm! Other than that every card that is "Something Charm" is a three or less mana instant with three modes of which you may chose one. There are 45 Charms presently and two honorary Charms, those being Hull Breach and Warping Wail, the only cards with the Charm formula but not the name. I'm not even sure Hull Breach counts as it is somewhat integral to its flavour that it works as it does and is clearly not designed in the same capacity as the Charms. Warping Wail is however 100% Eldrazi Charm and is almost shocking that it isn't called that! I guess Wizards might argue that another facet of the Charm is that they come in cycles which Wail doesn't but still!
While a lot of Charm cards no longer cut it they laid the foundations for many great cards styles to come. Commands, Confluences and the escalate cards now all outnumber the Charms in cube yet they owe their origins to the humble Charms. Modal cards are great in magic and even better in the cube. Obviously options are good, that is a well known thing. The difficulty arises in trying to assess the value of those options. How much are they worth in mana terms for example?
The many Charms tend to all be relatively playable with none really being overly powerful. This makes sense, you trade a little bit of cost efficiency for those extra options. Charm style cards are actually very hard to evaluate. There are lots of subtle and even counter intuitive scaling factors going on. You need to consider the whole card at once yet you also need to evaluate the modes independently too. Some failings on a Charm are negligible while others are more critical.
If you have a Charm where one of the modes is just so good you would play it regardless of the other modes, say a Boros Charm in a burn deck. In that list it is basically just 2 mana 4 damage which is better than the alternatives and earns it a slot. The other modes don't matter at all then and are just mild spice that may occasionally help out. This case is not the norm however, most Charms have no single mode that is above the curve and when they do it does tend to be in the more focused decks that are doing something a little different and/or linear such as the burn lists. Most Charms are either all a little below the curve for any one effect or have just one mode that is pretty close to being on par. In these cases you need at least two modes and ideally all three to offer something you want.
One of the real strengths of the Charms is the ability to play the narrow effects and this is strong because a lot of narrow effects can be very swingy. Combat tricks are a common example. A trick at the right time is a huge tempo swing, it can be like a Control Magic, at the wrong time it is a Lava Spike or even a blank. Narrow yet powerful effects on Charms with at least one generally useful mode go a really long way to improving said Charm. The narrower effects can afford to be further from the bar in cost as they will still more than pay for themselves when you are able to use them well. It is the more general effects you need to be closer to the bar with. You cannot have a Charm with one always viable cycling/card quality mode and two decent but super narrow modes if the always viable one is like a 3 mana cycle or something.
A good Charm has at least two useful modes of which at least one is generally useful and at least one other has great value or swing potential at the cost of being narrow. The good Charm will also have at least one generally useful ability fairly close to the cube par given the cost of said Charm. To appreciate this you need to know how much each effect on a Charm should roughly cost in your meta/format, how frequently each effect will be useful and the potential range on each effect.
A lot of the one mana Charms are simply too low powered to be useful, most one mana cards are not powerful enough as it is so when you pack them full of options and lower the power level you wind up with some super low impact and narrow things going on. Take Vitality Charm for example, the +1/+1 and trample would be a nice trick mode if one of the other two modes did something useful at all but a 1/1 is not worth a card and regenerating a beast is super narrow. Sapphire Charm is nearly playable with its nearly cycling for U and a bad Unsummon mode but it just isn't quite powerful enough again.
The bigger Charms are typically gold. This allows them to be more powerful both on the gold front and the higher CMC front. I am on a bit of a crusade against gold cards at present. I have found reducing their numbers to greatly increase draft consistency and helps to balance the meta. Too many gold cards and three colour decks are not just the best in my cube but almost a necessity. As such I have a single two colour gold Charm remaining in my cube despite a significant number of them being powerful enough for cube use.
Some of the older three colour Charms struggled due to poorer fixing back in the day. While most of them had a good set of abilities they were typically all too over priced at three. Few of them had a broadly useful mode worth much over 2 mana and not enough of them had situational big swing effects. There were far more generic things like a Boomerang / Terror / Shatter mix. Many would have made the cut back in the day if the fixing was good enough with slower lower powered things going on. Now they are not so viable on the whole despite the fixing situation improving a lot. Spells have generally not power crept over time but modal spells really have. We have seen this more outside the Charm (Commands, Confluences etc) cycles but it has still been noticeable within them.
So, the key points to evaluating Charms are as follow. Does the Charm have any one mode you always want that is better or as good as a card you would otherwise play? If the answer is yes then play the Charm and skip the rest of the evaluation! Assuming this is not the case then you need to look at all the modes and work out how much you want them and how often they will be useful in your list. You also then need to work out roughly how much mana you think the ability is worth, ideally down to 0.2 mana or so but to the nearest half is probably OK. You need at least two modes you will quite want. This has to be either something that is very powerful quite often or fine all the time. A third useful mode is nice and will further increase the value of the Charm but is really just a modifier at this stage. If you don't have at least two modes you will want don't play the Charm.
When you have two good modes for an archetype or more it is time to start working out how good the Charm is in those places. Unlike most other magic cards it is not possible with Charms to simply add up the value of the components and divide by the mana cost to reach some arbitrary value of power. Obviously you don't get to use all of the Charm and so adding up the value of the effects would massively overvalue Charms. Equally you cannot simply take an average of the value of the modes either as this assumes you are using each equally often which entirely isn't the case. It also fails to give any value to the option on doing any of the three which is a significant part of what makes Charms good. An average of the abilities value would be an OK starting point provided you are able to appropriately modify that result based on how well the abilities on the Charm complement each other. A Charm with complementary abilities is worth substantially more than the average of its parts while one without likely isn't.
There are several kinds of ways I would categorize abilities when considering charms. You have always useful ones, high potential ones, good and bad scaling ones. Good Charms have a spread of all these effects. Strengths of these different effects will mitigate weaknesses from the other types but not from alike ones. Always useful modes sound like the best but they suffer the most from being over priced. Card quality, draw, burn, bounce, and counter magic are all pretty universally good. As such there is loads of it available in cube and almost all of the focused effect cards are better than their Charm counterparts. People are expecting you to have those things regardless of Charms and so you struggle to get great value from the more mainstream effects on those modes from the Charms. While it is great to have at least one always or often useful mode on a Charm any subsequent ones add far less than you might think.
Izzet Charm is a prime example of this. All the modes are great, Spell Pierce, Shock effects and Faithless Looting all see a load of play in cube. All are generally pretty useful. The issue with the card is that all these effects are over priced combined with the fact that none of these effects really complementing each other enough. Yes, they all do great and different things but they all do it at too much of a cost. Izzet Charm could be made insane in one of two ways. Either it could Searing Spear, Negate or Tormented Voice as its options (wouldn't even have to be all three) which would be enough of a power increase to mean you didn't care about the lack of complementing as it has good effects basically on par for expected cost.
The other way to improve it would be to replace one of the three rounded abilities with a great scaling one or a narrow but really high potential effect. Perhaps replacing the burn mode with "deal damage to target creature equal to the number of lands you control" or perhaps "each creature you control gains prowess until end of turn". These are modes than can have much greater value than two mana but are not things you would play on their own as you would with Spell Pierce or Shock. These modes are far more complementary as the value of the option between them is far greater. The unplayable situational modes and narrow effects are made playable by the more consistent modes. The over priced consistent modes are mode much acceptable when they can do alternate exotic and powerful things, even if infrequently.
A different way to look at this is from the perspective of the individual card comparisons. Thoughtsieze is a very good card but you pay three times the mana for it in Mardu Charm roughly. That sounds awful when I am balking at fractions of mana over costings on effects but it doesn't take into account the whole picture. Thoughtsieze and that kind of discard effect have poor scaling into the game. Those kinds of narrowness are accounted for in the cost of cards with those singular effects. In essence, Thoughtsieze and the like are only able to be so cheap as they have a fundamental drawback. As soon as you take away the potential for a card to become dead you make it better. Any cube playable card or effect that is at all situational will gain far more out of being part of a Charm (or having cycling) than a cube card that is rarely or never dead. The discard mode on Mardu Charm is actually very good and often underrated due to the out of context comparison with cube staples.
Things are under costed in magic when they both single purpose are narrow. Oxidize is a one mana thing. In the same way that Thoughtsieze is undercost because of poor scaling, Oxidize is under cost because it will simply do nothing in a lot of matchups. This is the mark of a sideboard card in constructed magic. In cube things are a little more blurry. It is very nice to have access to some of the answer style sideboard things (rather than the proactive hosing effects) in your maindeck. When bringing in four Oxidizes from the board in constructed the fact it costs one is important. When you have those answers in cube you care very little about the cost. Sure, one mana would be nicer than three but if you lose to the Jitte or the Shackles or whatever if it stays in play then the cost of your effect is not something you will be complaining about.
To paint the picture of a perfect Charm we would have one or two abilities that scaled well into the game, we would have one or two effects that are generally useful. We would have one or two things that have reasonably high potential but are not the sort of thing you would dedicate a card to. Lifegain is a good example of this. Most slower decks want some to have game against red decks but no decks ever want pure life gain cards, they just want it as a free thing on an otherwise playable card. A pure lifegain mode on a Charm is a prime example of mode you wouldn't dedicate a card to but that you do want. Things that effect combat are the most common thing fitting into this category. Lastly you want at least one ability relatively close to the mark on mana cost value. It is convenient if it is your most playable mode but not essential for that to be the case.
The real trap is assuming that the more modes that overlap with cards you individually want leads to the best Charms. The most powerful Charms that offer the most are those that specifically don't have things you would play. Temur Charm is a great example. I have won and lost a lot of games as a result of the 3 or less power creatures can't block mode. You would never play a card that just did that and nothing else. Even at one mana that would be far too narrow to be worth a slot. On this Charm however it offers a lot of reach and frequently gets more than 3 manas worth of value.
While you can get an idea of a Charm's potential by looking at it's modes and considering all the implications mentioned in this article nothing compares to testing. With so many subtle variables all affecting one another a slightly wrong evaluation on any one part can have significant ramifications on performance. If you suspect a Charm fits well in a place or just looks inherently powerful and somewhat suitable to be played in something then go ahead and play it. Some are underwhelming, some are fine and much as you would expect but more so than with most cards a lot of Charms will surprise you. I plan to follow this shortly with a top X list for Charms. Initially in fact this was just the preamble to the Charms list but I quickly realized how complicated Charm cards are to evaluate compared to most other magic cards. The top X list will draw heavily on ideas in this essay.
Compared to Charms other multimodal cards like Confluences, Commands and escalate cards are not any harder to evaluate. Ultimately the only difference is that they have around 10 modes rather than 3! This may sounds like it is infinitely more complicated but it generally isn't. There is lots of overlap within those 10 options. Also you will recall that beyond the first two relevant modes on a card the remaining options just work to enhance it a bit. Imagine Cryptic Charm that had only three modes; counter draw, bounce draw, and tap all draw. This is most of what Cryptic Command does and would be a very powerful card indeed. It is very easy to analyze when looked at like this and using the ideas set out in this essay. Cryptic Command is a bit better than this imaginary Cryptic Charm but you don't need to go any further than working out that Cryptic Charm is playable to know that the Cryptic Command is a good card.