Yeah, I agreed they shall can not be targeted by melee attack when they are flying, and attack of oppotunity would still work on them. They don't just stay up 20 feets forever. I said so just to simplify the situation and place the flying creature in a position that not too far from ranged attack and melee attack can't touch it easily. They are still vulnerable ---- especially there are ton's of loots to throw on them.
If the programmer are willing to spend more time on this, they could set different height of flying seperately, from 5 to 30 feets,and a 100 feet for shotters, an untouchable height that none would be able to affect.
For myself, 5-feet-high flying plus 20 feet flying is already good enough. I only hope larian studio would listen to this and do something, anything.