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
Irenicus's Vile Duplication 1
Clone on a spell invites combos with the likes of Dualcaster Mage. Not much use beyond that.
Fang Dragon 2
Well the creature portion of this card is not powerful but it is at least quite threatening. It is all about the adventure here. If you want this effect then getting a big, if over cost, dragon on the back end of it is a pretty substantial upgrade. Mostly this kind of low impact mass removal thing is too narrow for the cube. There is a pretty quick meta shift when there are too many good ping effects or too many 1 toughness dorks. Even if this was good enough the shift would happen and make it not so. Impressive stuff for a common though. Great for tribal dragons as well.
Nemesis Phoenix 1
Powerful but again too narrow. The non recursion cost is way too steep meaning you need looting, but then you also need to be reliably attacking in multiples, all of which is way too much hand holding.
Noble's Purse 7
If this didn't enter tapped it would be one of the best mana rocks printed since Urza's block rivalling the power of Talisman, Signets and Pentad Prism. The latter of which is the closest comparison. This trades all the speed of Pentad Prism for greater convenience and return. Purse is three mana for two and is all colourless upfront. I don't think this quite gets there for cube as the decks that play mana rocks are the kinds of deck that want to spend a lot of mana every turn if possible making the three mana cap on this awkward. There are plenty of builds however where I would want this over Signets or Talisman for a wide array of reasons, perhaps broader fixing, perhaps the potential to get burst mana, perhaps the ability to get 4 artifacts into play with 1 card and 2 mana, perhaps simply some treasure synergy. This list goes on. This is a very strong card indeed with a lot of homes and applications. More constructed than limited and more synergy than raw power but despite all this a very rounded and playable card where ever you might wind up running it.
Dungeoneer's Pack 0
Sorcery speed activation is a bummer here. The card offers quite a lot but not enough for the speed and cost.
A little unreliable for my liking. You really don't want to do the -3 blind but you will often have to if you want more out of your card than a five mana Serum Visions... Also, the kinds of deck that can run this really don't want to be exiling their gas if they can help it. There is certainly a lot of power here, being able to gain +2 loyalty on top of five starting while hitting Serum Visions and reducing the cost of your stuff by 2 is nutty good it is very win more. Do that on the back foot and get beaten to a pulp. Will Kenrith does a lot of what Elminster does while being safer and more reliable and less narrow just being just the one colour. Elminster is a fun card but lack of ability to reliably protect himself takes him out of the running for cube.
Myrkul, Lord of Bones 1
This lacks threat, power, and consistency and is quite the laughable top end card.
Livaan, Cultist of Tiamat 1
Potential to scale but rather narrow and with far too low of a floor.
Lozhan, Dragon's Legacy 1
Five mana in two colours for 2 toughness and an ability that needs other stuff to trigger. Not common stuff either. Nope. Big ol' nope on Lozhan.
Lae'zel, Vlaakith's Champion 5
Great effect but unimpressive body. I do not love spending three mana for my support cards and if I am to do so I really want that support card to be a bit more impressive on its own. Certainly this is playable in most decks built around counter synergy but outside of a build around it likely isn't good enough.
Baba Lysaga, Night Witch 2.5
Priest of Forgotten Gods bigger sister. This is harder to setup than Priest and generally a lower tempo effect as well. You need to be pretty deep into sac effects for this to appeal but it does have reasonable power. A fun build around card.
Raphael, Fiendish Savoir 4
A bit low impact and high up the curve for me to want in a sac deck even if this gets very out of hand with a Goblin Bombardment or similar sac outlet! It is getting this going that is the trouble, if you miss that first trigger this card is pretty poor. I can see this being used in tribal decks as well as some iterations of more top heavy sac decks. Reasonable power, reasonably well suited and playable, just a bit of a top end middleman gold luxury card for me.
Oji, the Exquisite Blade 1
Cool effects but a little over cost and with slow returns. This looks playable but in practice I cannot imagine where I might consider this in a 40 card deck.
Mazzy, Truesword Paladin 1
A bit top end for what enchantress and aura decks want. Reasonable threat and value but will be superseded by cheaper cards in 40 deck decks.
Gale, Waterdeep Prodigy 2
Not as good as it looks. For this to work out you need lots of both instants and sorceries and ideally some way to put them in the bin beyond just playing them. You then need those cards to be proactive. With all those caveats you are basically playing an Izzet deck chock full of burn and cheap cantrips with a smattering of spells matter dorks to take advantage of all that support. Even when you have all these things going on Gale still needs to live and he is only getting you a bit of card advantage when he does. A fun card and reasonably playable but I cannot see it performing all that well all that often.
Astarion the Decadent 2
Too narrow and top end but none the less quite dangerous. Astarion can just finish people out of the blue. No one really expects face damage outside of red. I am sure there is a Tooth and Nail kill combo with this and something else that does half a life bar and that still probably isn't worth including anywhere. If he isn't burn he is probably a bit small, slow, conditional and vulnerable.