We allow casting spells (including balls/bolts) while flying, but melee or missile is another matter. Is it powerful? Yes. But you still have options, like firing missiles/bolts/balls at the flying mages, or dispelling the fly spell, or being a flying foe yourself. And since casting spells after using 25% of your PPs require SCSMs, which are rather hard "in a melee environment", it's not that overpowered (after a couple balls/bolts, you're more or less "out of ammo").
Also, a pure caster on the ground is quickly disposed of by any melee-fighter, who can strike from 50' distance using react and melee (sure, they get lots of minuses, but against a pure spell user you don't really need much OB left to hurt them). Combined with no spell casting while stunned, that sure makes casters on the ground really vulnerable! Getting airborne as fast as possible is their best bet for surviving - but then, of course, they are easy targets for enemy bowmen (who can now fire at them without risking hitting one of their own). Sure, if they are high enough level they can maybe deflect one (or maybe even two!) missile(s) each round, but they'll definitely run out of PPs before the enemy runs out of arrows, and since they can only cast one spell each round, they can't do much else while they're deflecting arrows...