Not really a balance issue, but...
Gate of the Sun on the hardest wonder difficulty, does not in fact, allow you to work mountains, despite saying it gives them yields which in turn implies that it would let you work the mountains it gave yields to. As it requires both a mountain and a lake to be built when on that difficulty, it feels real bad.
I've tested this across numerous games over various versions and never did I ever get the ability to work mountains and thus didn't get any of the yields from them that the tooltip promised.