Jace is the type of card that's so simple players don't feel bad loosing to it. IMO that makes Jace a good choice for Cubes that normally wouldn't play cards as powerful as he is.
Part of the reason (some) players don't feel bad losing to it is that they don't
know they're losing to it (again, imo) - like
Phyrexian Arena, but with loyalty.
Is Jace really that powerful? I've played against him in a cube with a power level compareable to mine (maybe 5% higher, but not much more) and it felt not too crazy. Isn't he just a slow draw spell if you don't get to the ultimate? Like, I feel like he's not above stuff like
Fact or Fiction really.
The ultimate being written on the card is about the entire power level you get out of it - when it kills, it really didn't, because you had an unanswered Jace for 4+ turns and could have just been -1ing - but I think it's probably... around as good as
Fact or Fiction as a card? Maybe slightly better, 3 mana is so much better than 4, and it provides a different style of "opponent has choices that impact how many cards you get" that I think is a little stronger. But that's not me insulting Jace! I think the other four shown are cards I would all put far below FoF. Maybe
Elspeth Tirel would at least be in the conversation, but I'd put her several rungs below Jace.