Put simply...there is no way to make a flying unit work effectively and realistically in the game engine. It just isn't designed for it.

Archers will fire ( and hit ) the base of the unit....and you can't stop that.
Soldiers will attack it's base
Units will be affected by it's passing
Terrain such as rivers WILL stop it, just like a walking unit.

Can't imagine a unit of dragons being held up by a bridge......

You CAN make a unit which appears to be up in the air, and you can make the animations to make the flying unit attack the ground....but it's a LOT of work and would still result in a unit which the REST of the game units didn't respond correctly to. Which is why no-one has bothered.