I dont think air as a unit is OP in terms of its strength. The problem with air is how it gimps the person trying to counter mass air. Now, this wont be much of a problem for the top tier players who can manage the game and their economy very well. But for average players, countering mass air gimps your tech up ability. Massing air costs no power.
So, you could mass air, force the opponent to keep wasting their power building AA to counter.
Then, you pull out any of locusts/wraiths/hunters/rangers and rinse everything they have with no contest.
Note: This is for AVERAGE players. This wont happen to a top tier/skilled player who knows the game very well. Point is, it is very easy to mass air, but harder to BEAT it AND destroy their bases with your gimped AA army, especially as unsc (as banished locusts compensate for this).
I know that by the metric of unit interactions Air is not OP. The problem comes with the cost and level of skill required to counter it without having your enconomy and base killing potential gimped. All the air spammer has to do is change to vehicle/infantry army comp and they win due to you dedicating a chunk of your army defending against such air spam.
Again, I am an average player and know that Air as a unit is not directly OP. Its the economy effect of countering vs building it that is the problem. I am no expert. I dont have any ideas or solutions. I am still re-learning this game after being away for nearly a year.