0 - unplayable in 40 card singleton
1 - effectively unplayable
2 - has low tier constructed decks it might go in
3 - has mid tier constructed decks it does go in
4 - pretty powerful stuff with several potential homes, able to perform well in lower powered cubes
5 - powerful stuff that is either just too narrow or has too many superior alternatives
6 - fringe cube worthy
7 - cube worthy
8 - cube staple
9 - unpowered cube bomb
10 - powered cube bomb
Migloz, Maze Crusher 5
High power and high utility. This is a whole lot of stats for the mana and can grow, evade, or even Disenchant. Practically this is a little riskier than it looks as it is still very easy to fall behind in tempo playing this to no immediate effect on the game. As far as cube goes it is not at all the kind of card I am looking to include in the gold sections either and that is what kills it. As a mono card, red or green, perhaps even powered down a little, this would be very much in the cube. Taking up a gold slot however and it isn't adding enough to the environment for the high opportunity cost of playing it.
The Seedcore 2
This is pretty narrow, a kind of Pendlehaven for mites! I am only playing this in a deck with both some phyrexians and the ability to give my opponents poison counters. Even then the cost of it sometimes/often being colourless is large and so we are a long way off an auto include in the only place you can play this.
Argentum Masticore 2
This is pretty polar. Fail to answer this and it can chew through your whole board while beating the bejesus out of you. Alternatively, Shatter this for fun, or Mind Twist their hand away and watch as they put this in the bin, or just watch them put lands in the bin as they fail to tread water with this expensive road block. The risks here are high and so this is not a tool I am interest in playing with even if the ceiling has allure. Even the protection makes this more polar. It is the sort of thing the card needed to not blow but instead of being a delightful ward 2 or 3 it is a filthy protection from random thing which leads to poor play experiences.
Venerated Rotpriest 3
A very powerful one drop indeed. This is the kind of thing that really pushes the viability of constructed poison decks. An auto include in any 40 card green based ones. Not the kind of thing we want to play with or even encourage in limited setting however.
Zenith Chronicler 2
Because gold based decks are so rampant and powerful we needed more things to hose them? Pretty sure this is just a cute EDH toy.
Red Sun's Twilight 0
Too narrow for maindeck anything and not even a good sideboard card. If you want to kill lots of artifacts you want to do it cheaply and there are many tools far better than this for that.
Venser, Corpse Puppet 2
A very powerful magic card if we are adding the parts together but the need of proliferate and the fractured nature of poison cards keeps this pretty fair. It is going to be near impossible supporting this with enough proliferate in cubes even if you wanted to and you really shouldn't want that!
The Mycosinth Gardens 3
Cute little fixer than can jump ship and go become something else. Some utility, some potential Stalking Stones style land threat. Not a bad card but too aimless for cube although I am sure this will find homes in all sorts of places being quite the unusual card.
White Sun's Twilight 1
Seven is a lot for a Wrath, we can Wrath and put stuff into play for six, and we can put stuff into play that isn't massively narrow. This seems to pricy for poison decks to use for the tokens and for control decks to use as removal. That all feels like it leaves this with no potential to speak of.
Kethek, Crucile Goliath 2
A fun cascade style Birthing Pod effect. If you can abuse this with prototype or affinity effects that let you cheat out high mana value cards then Kethek can be scaled up but is that worth it? It isn't reliable or all that exciting in cube and seems like too much effort elsewhere. This is fine and fun but also narrow and fair with problems over consistency as well.
Skrelv's Hive 6
On paper sounds good but in practice not great I suspect. It reminds me of Dreadhorde invasion which was vastly worse than Bitterblossom. Tokens than can't block are weak, being 1/1s doesn't help them attack well. They are only effectively 2/1s if unblocked and able to do those 10 hits themselves. The lifelink is late in the day at best and pretty low impact. You would do well to get back more than the life you paid for each mite. All in all I don't see this. White is the colour of Anthems and equipment so I do need to try this. It is a very cheap source of ongoing fodder. It is however so much worse than Bitterblossom that white has a whole lot of contextual benefits to make up for. White has plenty of ways to churn out tokens as well so it has to really want specifically the things this offers, that being no ongoing mana investment, reliability, low upfront cost, and relatively hard to remove. Otherwise it is just going to play things like Precinct Captain, Spectral Procession, planeswalkers, Castle Ardenvale, etc, etc.
Kemba, Kha Enduring 2
Not enough support to be worth running this in cubes. Might not offer enough punch to find a slot in equipment heavy decks. It is the all in one body and Sigarda's Aid in the worlds of Colossus Hammer but even there it seems a bit awkward.
Urubrask's Forge 6
This is a slow threat but it does a lot of things well. This is impressive reach that needs an unconventional answer. You cannot Wrath this away or Fatal Push it. You need a Shatter effect or at least a willing first strike dork to live and remain on defence. Forge also scales up both by itself and if you happen to have any sort of sac outlet to pair it with. Red is not at all short of threatening 3 drops but this one does feel very well suited to cube even if it isn't packing quite the same power levels as the best of the red three drops. Six turns in play affords this 21 points of damage. Slow compared to comparable creatures but far harder to stop, both in combat and with removal. Far more relentless but equally zero defensive properties. Not always a problem in red but noteworthy none the less. I think this will be pretty good when it is played but not get as much play as it needs. It is a bit linear for slower decks and a bit slow for the aggressive ones.
Evolved Spinoderm 3
Someone wanted to revisit the classic Blastoderm and came up with this. Considering how much better dorks have gotten since Masques block and given this is a rare not a common it is really not much of an improvement on the original. This is meaty and gives you some time to safely buff it up if you like. By itself it is just fatty limited to three attacks. If you can sneakily find ways to oil it up then it could be devastating but as it stands I am not too interested. I like my threats to stay in play by themselves. Hexproof for a couple of turns is nice but without a way to properly abuse that I am unimpressed. Really this is just another polar card. Sometimes it is utterly negated by some Baneslayer style card while others it is unstoppable and does critical damage by itself.
Ovika, Enigma Goliath 2
Big and gold and thus already somewhat out of the running for cube. Ward helps but not quite enough here. You really want to be untapping with this and still have some gas in hand to deploy. That is too much to ask and so we can move on past this very powerful fatty.
Mercurial Spelldancer 5
They might as well have called this a hoop jumper. This is like a really bad Glint Sleeve Siphoner. It takes a billion years to gain any advantage and mostly dies before getting there. You also need to play this in a deck with spells that are useful to copy, be that ones that draw cards or ones that kill things. Try playing this with mostly counterspell effects and you have a sad card. This is fine, the unblockable aspect is relevant and the ability to actually block adds a good amount of utility to this card which many other similar such cards lack. This probably plays fine in cube and could hold onto a slot if there were more in the way of aggressive and evasive blue decks.
Tablet of Compleation 2
Way too slow for ramp and way too slow for draw. Slapping both together doesn't do much to help with this speed issue.
Geth, Thane of Contracts 2
This is likely just unplayable in cube with it being so hard to build with. The tempo hit from this is real and the upsides are hard to really abuse. Either play Recurring Nightmare style things, or Zombify type things. I wouldn't want this anywhere, it is either too slow, too vulnerable, or too detrimental to my own plans. The best thing about Geth is just being a 3/4 for 3 and thus an OK body to flop into play. He might then have some cute utility with your many Nekrataal effect dorks. Sadly that upside is just paling in comparison to the downside of giving your stuff -1/-1 on average.