Is there a "gotcha" with Caged Sun?
I think the big drawback is simply the cost. If you hit Gauntlet of Might on turn 4, you can play 8/10-cost cards on turn 5. That's pretty strong, and much stronger than getting 12/14-cost cards on turn 7, at least in non-multiplayer games.

In general, you want your mana acceleration to be as cheap as possible, else by the time you can play it you don't need it. That said, Rise of the Eldrazi does bend that rule.