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
Modify Memory 1
Too expensive for this effect in 1v1 play.
Eldritch Pact 3
A potential finisher or just a big draw spell. I most want to play this in a deck with Titan's Nest! This is certainly narrow but it is also interesting, scalable, and powerful.
Neera, Wild Mage 1
Cascade fun times! Uncapped cascade in terms of mana too which can get naughty and indeed has to do so in order to be wroth doing what with you giving up the original spell. Even so, the six mana price tag on what might as well be a wall in terms of board presence, renders this a fun EDH card, even with the combo potential to slap an Emrakul on the top of your deck and play it immediately with a Gitaxian Probe following Neera.
Topaz Dragon 1
One of the worst of this cycle of cards. Unimpressive at both ends although at least the adventure is an instant giving it mild combat trick vibes.
Stonespeaker Crystal 1
Where Sisay's Ring and Relic of Progenitus meet. Too costly to be utility and not efficient enough to be ramp.
Young Red Dragon 6
Love Bathe in Gold. That is a great way to spend a couple of otherwise wasted mana. Great for control, midrange, and tempo decks. The back end however isn't very efficient, even if you ramp into it with the adventure. Luckily it is relevant thanks to the flying and is cheap enough that you will expect to cast more often than not. Neither half of this card brings much power but they are both useful and playable. This has a chance of making it. It will help sort out your early game, ideally ramping you into something potent ahead of curve to pull you ahead allowing you to then continue pushing that advantage with some late game gas. The dragon is enough of a card to make the adventure somewhat free and thus all rather appealing. Sadly I think the raw power leaves it failing to compete all that well in cube without a bunch of empowering synergy at which point it is going to be too narrow.
Thriving Gate Cycle 3
In terms of fixing we have things very similar and things functionally identical. Great for budget places and good on space economy but not powerful enough for most cubes. What these do that is interesting and useful is bring the total number of gates to an impressive count allowing for an all gate mana base should you wish. Not the most powerful gates we have seen but none the less a significant contributor to the archetypes massive rise in power with this set.
Heap Gate 4
An untapped gate, a fixing gate, a treasure producer (and thus storage lands of sorts). This does a lot to the point you might play it and other gates in a non-gate deck. Mostly just more good gates to get you going in your gate archetype.
Swashbuckler Extraordinaire 2
Some nice utility here but without other treasure generators this is not enough. Even with them you still need good dorks to empower with this resulting in an overly narrow card. Yes, it is broad in application technically but that average performance level is not sufficient to get it near a cube deck and so it requires supporting to boost that power level which is where it becomes narrow.
Mystery Key 0
Card draw does not want strings attached. Having a dork that gets through is way too much in the way of strings rendering this unplayable despite an OK power level.
Underceller Sweep 2
This is powerful and a lot of value but it is too slow for the kinds of places you might consider it.
Traverse the Outlands 1
This has impressive scaling but it is too conditional and too late in the game to be of much use. Ramp wants to be earlier in the game.
Taunting Kobold 2
Lack of any evasion means this is rather a one shot affair most of the time making it even less powerful and power was certainly the thing lacking. A cheap and unique card resulting in something that might get play despite seeming objectively useless on the face of it.
Windshaper Planetar 2
Comically flash is so good that a five mana 4/4 flier with it looks reasonable. The effect isn't totally dead either, you might get to save a planeswalker. Still, neither exciting or quite powerful enough for cubes.
Caves of Chaos Adventurer 6
Quite a punchy body with some immediate value and then both some value and threat there after. This would do OK in cube and it looks reasonable power wise but I fear red simply has too few slots at 4 mana and they are filled with some of the most impressive and scary threats.