Agreed with both
@sigh and
@blacksmithy.
As such, this isn't going to sound too different than their approaches, but to me, "fair" means playing Magic with opportunities and outcomes within a relatively tight band across all players. Cube is already a great equalizer for those that don't have a lot of funds to dominate a beer-filled EDH table or the local Modern event, and the draft portion helps self-correct even further. But if there are cards that allow for a specific player to reach an outcome that is meaningfully more powerful than what a typical deck could hope to do, that imbalance creates an unfair situation.
Now, dedicating the resources in-game towards a specific goal or self-constraining deckbuilding around maximizing a strategy are wonderful, and don't really factor into fairness, unless the strategy gives a player undue ability to "cheat" on resources.
From this, I would consider cards like
Tinker unfair, while
Balance is technically fair, but simply too high of a power level relative to the cube experience I otherwise support.
Dark Ritual could be fair, as there's a real cost to use it, but when combined with
Mind Twist, it's anything but. I guess I'll just go back to
@sigh's definition -- a card or strategy becomes unfair when it bypasses the game engine's core constructs. Fast mana, cheating on costs, and taking away interaction fall into this category, but it's a gradient on each, and a lot of the most apparently oppressive cards in my cube are diluted by its size.
I target my own cube as a "Fairmax" list because both my players and I like to play with the most powerful cards in Magic, but to do so without consideration makes for uneven games that surpass our tolerances for variance in both opportunity and outcome that we'd want for a casual game night. This is especially important for us because our cube is 720 cards, a necessity due to our playerbase, which loves to randomly have 14 players some nights, while offering greater variance on nights where we scarcely manage a pod of 6. Sure, we shouldn't run
Grim Monolith (it's the last holdout of fast mana), but slowly but surely we've been picking out anything that fits into the first "unfair" category outlined above. Cards that are simply too strong relative to the environment, yes, could be considered "unfair" in context, but I typically try to avoid that terminology when it's context dependent.