I don't think flight is overpowered,
unless the whole party has it.
So, one PC can fly. What about the other five? The Aarraacokrraaraaa (is that enough a's and r's?) can fly over obstacles outside, but the rest of the group still has to slog through the swamp. All that achieved was is to split the party. And what is rule #1?
Sure, the flyer can get to the top of the cliff, but unless they have sufficient carrying capacity, that won't help the rest of the party who still have to climb the rockface. If the flyer has placed ropes at the top, though, the party's climb will be easier.
If a flyer can target a foe with missile weapons then the foe can target the flyer.
Outside, flight is, however, excellent for scouting. However, a wizard's familiar can do that and no-one calls for flying familiars to be nerfed.
Except the invisible ones, dammit, quasits arrgghh ruining my GM plans mutter grumble.