Nahiri, the Unforgiving 7
Finally a three mana walker in Boros colours. Just waiting on Selesnya now to get one of these. Nahiri isn't all that in terms of power but it is none the less pretty welcome. Boros colours are fantastic at supporting walkers, and indeed the payment of life costs if needed! Nahiri is decent at protecting herself with a high loyalty count and a +1 that ensures the most threatening dork in play attacks your face rather than hers. Mostly Nahiri is a loot station, or rummage station, that powers out any other yard based stuff while ensuring good consistency. If good consistency isn't enough to take the game her 0 ability should be able to apply a good amount of pressure in the mid to late game. I think this is a weaker card than the Royal Scions while being fairly similar overall. Even so, I think Boros hungers for this sort of thing enough that it will last decently in cubes. I like the card overall too, a nice design for a Boros walker and a nice safe way to implement a walker that can cost three or four mana.
Mondrak, Glory Dominus 7
This is going to be a valuable card in this set. Being an Annionted Procession is a big deal for EDH and one that can be a commander too! Popular this will be. Powerful it is too, such that it may well wind up being a player in standard. It is cheap to turn it indestructible and can be done in response to things giving this some real game. It is a bit the best of both worlds, you don't have to use the ability but it will often feel like you have regardless. Double tokens in cube is nice, it works with a lot of stuff but it is too narrow and effect by itself. The big deal is coming on a meaty 4/4 for four that can become indestructible. That all adds up to being a lot of card. Very reminiscent of the gods from Theros. I expect this to be a solid cube card and a fun one. Play wise this is probably on the line as so many cards are, it is the fact that it is fun and cool too that will help this last in the cube.
Jace, the Perfect Mind 7
Awkward. Mill cards are not balanced around 40 card lists and as such play all kinds of wrong. This Jace is threatening because it can mill out way too easily. As a means of drawing cards or controlling the board this card is fine. Perhaps clearing the bar for cube power level but only just if so. It is the mill three and the mill 3X that will just make this a very tedious card in cube. It will end a lot of close long games out of nowhere and so I am disinclined to even test this. Sure, it is probably fine, an OK and rounded walker that can also just win the late game. If you want to force fast acting games then this will be fine, it might not even be good enough. If however you want nice close games they will often be slower and this will not enhance your experience. Probably this is a little worse than Narset (3) and a little better than Architect of Thought.
Slaughter Singer 2
Build around toxic low curve support. Good in that one potential place... A lot of cards will be coming in that category from this set it would seem.
Annex Sentry 5
Nice high toughness is cool here. Hitting multiple permanent types and only being a single white mana all appeal as far as this kind of dork goes. The toxic is fairly irrelevant. The cap on CMC for exile targets however is not and is likely too problematic for this to get much cube love. The card is fine and will hit most things and do what you want it to a lot but those key times you really need to clear a big Glorybringer or whatever and can't will just render this unplayable in contrast to the alternatives, despite the perks this comes with.
Experimental Augury 7
This might be peak excitement. It is a proliferating card quality spell. It is all my favourite things and it is reasonably well pushed. Anticipate is fine, it is playable. Instant speed proliferate can do quite a lot of work. The problem with most proliferate cards is when they fail to do any useful work the floor of the rest of the card is unacceptably low. Here it is not and so we might finally have a cube playable proliferate card! Happy days. This is no bomb, and it is quite polar. Blue is also a trickier place than most to get much back from proliferate, so this is not a card for every cube. I am a bit too into my proliferate to be cutting this anytime soon. Much more the sort of thing I will try and support more so that it looks better!
Bladehold War Whip 6
I don't much like this but it does seem good in a very cube sort of way. A 3 mana 2/2 doublestrike is OK. Below the curve but not by a lot and very threatening and on theme for a Boros card. The equip text is minor but the ability to reequip once your token is dead is significant. It will let you turn a lot of things into real threats. I am probably slightly overrating that as I am old and mana sinks used to be hard to come by. These days paying five mana to give a dork doublestrike is a big old risky investment and not exactly an efficient one. All in all this can't have the power level to make it, especially being cheap and gold, but it is certainly very close to there and should absolutely be looked at for those budget cubes.
Atraxa, Grand Unifier 5
Somewhere between Niv Mizzet Reborn and Muldrotha the Gravetide exists this Atraxa. In some cheat in decks this will do a very good impression of Grisselbrand. Even on just one hit Atraxa will hit fewer cards total than a Griseldaddy pop but she will hit some gas a bit more reliably, and critically not at life cost. Atraxa is too pricey to be good in cube if you are not cheating her into play. She is too much value and not enough board control or finishing power for a card that far up the curve, let alone that gold. EDH is obviously the main employer of this card, with a little bit of moonlighting in cool combo decks going on as well. Also - battle eh? Buffs to delirium and Goyf incoming! Lets see quite how cheaply we can cast an Emrakul for etc...
Sheoldred's Edict 7.5
A two mana Edict given a quite necessary push to bring it in line with current removal needs and standards. Soul Shatter had just about become too weak for cube and Sheoldred's End looks to be a reasonable replacement. It is not better as such, just being cheaper is a big deal. This Edict is a good planeswalker removal tool and a great early game removal spell. It is in the mid game where you will have to work a bit at having it be good removal. When you need to kill a 3-5 drop but all this is doing is killing a 1-2 drop then your card is a sad card. Soul Shatter didn't have a very high ceiling but it kept a reasonable floor thanks to the highest CMC stipulation. Sheoldred's Edict has nothing like that and so the floor on it is still a bit shakey. I do not see myself using it to kill tokens very often but it is nice to be able to specify. Mostly it is nice that there is a non-token creature mode. It is the prevalence of tokens in cube that has kept most Edicts non-viable. Removal is certainly increasing in value in cube with the utter brutality of most creatures. The ability to select and tune your removal suite is welcome and this will help a lot on that front. Getting around ward and other protection effects was always one of the biggest draws to Edict effects, and those protective abilities are on the rise alongside removals increased popularity. I expect this to get enough play and perform fine. It is not that much better than the many iterations of Go for the Throat, if at all. It is however sufficiently different and interesting without having to compromise too much on anything to be a popular cube card.
Seems clean and solid. This is On Thin Ice (or at least the way I allow the use of snow land in my cube) but it has the ability to hit planeswalkers too. This isn't top tier removal but it is very playable indeed. It is likely better than Fateful Absence for a more proactive deck and likely a little worse for a control deck. What is so good in cube about removal like this is that while it undulates above and below a card like Fateful Absence in power level based on where you play them you can still play these cards in all the various archetypes. Cheap, broad, reliable removal. Fantastic. Yes, this can be undone with the very few instances of enchantment removal in cubes, or the even fewer instances of things that blow up basic lands. And no, it is not instant, or flash or whatever. For a card this cheap and broad however I would not expect anything like that. This just gets the job done and lets you carry on playing the good game of magic. The card itself isn't good and exciting but it lets you have your excitement with your other good fun cards and so that is a win. Dull cards - the unsung champions of good games.