Yeah, in a dungeon/cavern/ruin environment, flying is less useful. Also true of a forest environment, when things can fly. In an open field, flying can help you escape, but bows and crossbows have very long ranges...
Do keep in mind the nature of the opponents. While random encounters might let a flier get away, any group that knows of the PC's power or ability in a vague way is going to prepare for flight -- flight is a reality of the D&D world, and things are built taking flight into account in the more populous or magically powerful or air-elementally related zones.
Flight is no real worse than, say, a monk with ungodly movement speed. The same strategies work against them -- things that limit their movement, things that slow them down, things that weigh them up (many fliers cannot fly with anything more than a light or medium load). If there's a critter with wings in the open, bust out nets, bolas, tanglefoot bags, ranged weapons, and your own spellcasters with fly.