I maintain that Balance is fair and... some synonym for balanced
I've found Recurring Nightmare to be remarkably tame - it's hard to use in the Vintage/Legacy Cubes on MTGO and in other formats you don't have as much uncastable nonsense to cheat in with it
I feel the same way about Skullclamp too even though my Cubes are very heavy on token makers etc; I think it can be a cool engine card for those decks especially as just accruing card advantage isn't the guarantee of victory it felt like it was a few years ago
...
Fractured Identity is incredible - LSV calls it the best non-power card in the Vintage Cube - but unlike the other cards here it's primarily reactive. If I'm stumbling or haven't developed anything good on the board yet, Identity isn't going to slam the door on me. Identity is also amazing at enabling great comeback stories and winning games that seemed unwinnable. It might be 'too good' still but on a different axis than these other cards
Upheaval is a problem thanks to other problems - namely fast mana like Grim Monolith. By itself it's a slow and situational card that can upset the apple card unlike any other card if the game goes long enough
This very strongly mirrors my experience.
Balance,
Fastbond,
Oath of Druids have been exclusively fun and healthy for us. All three of these cards are unusual in the drafting portion (they change the evaluation of other cards you open quite a bit)
and create unusual play patterns.
Recurring Nightmare and
Skullclamp have likewise never been oppressive for us. They're good cards if you can support them. I suspect this depends on how the creature and combat orientation of your cube. I've especially loved Skullclamp. It's such a cool engine for decks that otherwise wouldn't be able to chew through card advantage. Right now I never want to cut it.
EDIT: additional note, I've found
Library of Alexandria to be very medium. You'll definitely play it, but it's not particularly sought after.
On the other hand,
Fractured Identity and
Upheaval have both been very very pushed for us in terms of power-level. I cut Upheaval but not Fractured Identity for the reasons Dom highlighted. Upheaval was ending the game on the spot 99% of the times it was cast. Fractured Identity is maybe higher in raw pick-order but because it's reactive it serves a fundamentally different role that my players are happy with.
Compare the following two decklists which share some surface similarities:
Artifacts and Upheaval:
Stasis w/ Fractured Identity:
Every match with the Stasis deck was long, tense, and interactive. Fractured Identity was cast a lot (and snapped back a lot) but the games were still uniformly close.
The artifacts deck had the potential for back-and-forth interactive magic but instead ended with: "Alright, Upheaval?" "I concede."