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
Urabrask // The Great Work 4
Certainly a powerful card with some nice scaling. A very big Electrostatic Field come Birgi. The issue is that the spells matter archetype really isn't about synergy cards that far up the curve. Even three mana is a push for stuff that is triggered like this. Just a 4/4 first strike is not close to playable and a lot of the time that is what this will be, not awful but not great. The mana will not be too useful and you will not stand to get too many ping triggers. Even if you were assured one a turn that would look pretty poor. Sometimes you will flip this and get a bunch of value and others the flip will be a detriment as you will lose a 4/4 first striking body on the board! 3 damage to all their dorks will thin the ranks but it will not handle much that is actually a problem. Urubrask will be fine if you put him in cube but there are plenty of better options. There are better cards in this set alone that I would play before him and that I still don't think quite make the cut.
Scorn-Blade Berserker 2
Love it but not really sure what I am doing with it or where I am playing it. It isn't a good use of mana and the synergy isn't great. There are not all that many cards I want to put a counter on then sac. Perhaps modular stuff and Hangarback Walker. Mild potential then in a black Hardened Scales list? Nothing for most conventional cubes though.
Invasion of Amonkhet 2
Not terrible value but not the kind of card you can play in cubes and expect to win that much. Three mana is so much for a non-tempo play. Such things need very high power and suitability and this has neither. Just playing this pretty much ensures you are not flipping this! Or at least, if you can, you had already won.
Marshal of Zhalfir 3
Very solid Knight lord. That being said I feel like Mardu is the traditional home of the Knight and so this may get a little less action than it otherwise might if the colours were more aligned.
Meeting of Minds 7
Convoke is a real cost, it is not a cost reduction. It is perhaps more akin to delve than other cost reduction mechanics, such as affinity. Even so, I am reminded of Thoughtcast here. I want to build a deck that chucks out dorks and then I want to tap those dorks to draw cards and carry on the cycle. A couple of Iconoclasts, Sai, Young Pyro etc and this can become self sustaining to a degree. Sounds like an incredibly fun way to go about going off. Instant is a nice perk here, you can block and then play this making it a much wider application card. It still has all that cool proactive and combo potential but can also be slung into a midrange deck and used to good effect while on the back foot. That makes it viable for cube play. Even though it is unlikely to ever reach its ceiling in cube it likely has enough of a floor to make this solid.
Seed of Hope 7
Another banger. I do always get hot for the premium card quality stuff and this seems pretty premium. It is Sleight of Hand with multiple upsides and one downside. Overall I would say the perks, at least in cube and in green, outweigh the downside. That downside is simply that you can only find cards that are neither instant or sorcery. For this restriction you get two life, instant speed, and the milling of a bonus card into the bin. I think that all puts this around the level of Oath of Nissa at base but with more scaling potential in my current cube meta with yard stuff being more relevant than the synergies Oath offers. Seed of Hope is not quite so assuredly playable as Abundant Harvest but it probably performs better when you can play it. Seed of Hope is the best card quality spell on offer when you have strong things keying off the bin and no other instant or sorcery cards in the deck. With minimal yard based things and from around 10 of the 40 cards or so in deck being misses Seed of Hope stops being playable. That is a pretty decent range and should leave this being both suitably playable and suitably powerful most of the time. Green is the place most needing of card quality as you need a good ordering and ratio of things when you are ramping into cards. Seed of Hope should combine really well with the handful of other premium green card quality spells in cube in bringing consistent consistency where it is most needed!
Stormclaw Rager 5
A fairly rounded little sac outlet. If you want to support Blood Artist synergies in black you need some rounded sac outlets and you need a few more than the ones that are already cube worthy by themselves. This is an OK card that is just what that kind of archetype wants. A bit boring but it gets the job done and it is at least drawing you cards! I prefer the utility of my sac outlets being instant, ideally no cost either and so I'll shy away from this one and stick to things like Spawning Pit instead to make up the numbers!
Progenitor Exarch 3
As a three drop the power is pretty shoddy, I almost think it is better as a one drop! It compared very poorly to Blade Splicer both for synergy and for tempo. At five mana we get a bit more enticing on the power and seven mana starts to look rather muscular. Even so, those are pretty late game, really I want power earlier than that, or at least game winning power late. This looks weak compared to Emeria's Call or Ultramarine's Honour Guard, or Student of Warfare. These are all just more utility and more power options that share a scaling aspect with Progenitor Exarch. I want to like this card but I don't really feel I even need to test this one, it just seems lacking.
Shatter the Source 2
Lots of kick and lots of utility but the cost is just too high here.
Ichor Drinker 1
Not enough specific synergy or power to really seem like it is getting play anywhere.
Enduring Bondwarden 2
I want to like this but it just seems kind of bad. The 1/2 body with a counter seems a lot better than the backup and it is hardly something to write home about. It doesn't do enough on it's own, the power nor synergy are that high. If you can stick it on something that they have to block, turn that into a trade, then get that counter on this guy then you have some reasonable value, but that is situational to say the least! A cute combo with the black one drop backup dork but that is a booster draft thing not a cube thing.
Hangar Scrounger 1
Pretty poo but some potential for combo here, likely one for pauper or something!
Flywheel Racer 5
Cute Springleaf Drum come vehicle. This might be low cost enough to represent useful ramp therefor also having enough action as a card overall to be worth playing. Power is low but this does desirable things on the cheap.
Mirran Banesplitter 6
Bonesplitter come combat trick. To get this into action is half the price of a Bonesplitter however one re-equip and you are a fair way less mana efficient. As this has no toughness component you have to land it on a dork with firststrike or something before it is turning a chump into a survivor and representing a likely profitable combat trick. If you are just turning a chump into a trade then you will have to re-equip to gain any further value. Red does have a nice bunch of double-strike dorks for which this is especially scary. Menace pairs reasonably nicely with this too. Kari Zev is going to utterly end some peoples day with a Banesplitter! Although I think the combat trick element of this is some mix of too situational or too expensive overall I think that the +2/+0 for a single red with very little risk is very possibly worth it just as a source of damage output. Works super nicely with prowess dorks too. I think it will be very easy to average over 4 damage with this when it is in opening hand. Lots of utility, lots of very cheap damage potential. Mostly just worried this is a little bit of a non-threat, non-answer card. You can't run many of those and this is certainly not a very flashy one. This is the kind of card that shines when the cube has a polar meta with aggro decks going as all in as possible. My meta is presently the opposite of that with a very midrange feel to it. I want this to work out but I fear it isn't the time for it.
Akki Scrapchomper 2
I want to love this but it costs too much to use, is really only useful late, and does too little early. Great thing to do with old food and blood and treasure but again, not till late, and then not with a one mana 1/1 once per turn thanks.
Sabia Crytomancer 3
I like this but 2 mana is too much to hold up as protection. Perhaps you can play this in a deck that also quite happily wants a 1/2 flash hexproof dork, but those are not things you find in draft.
Ephara's Dispersal 4
Very close to good enough but I think pipped at either end of considerations by Repulse or by Fading Hope.
Realmbreaker's Grasp 3
How far Arrest and Pacifism have come, and to remain as yet unplayable!