But if you still have movement options available in the air after you airtech, can't you just airdash away or double jump after you airtech to avoid getting hit by an air-to-air or ground anti-air? And what about Ryougi's airtech counter option which has never been used in real play?
When you have air movement available after airteching and a decent air moveset in the air, the opponent usually will have to respect a good, unpredictable decision on your part; making the airtech still fairly strong. If the oppnent beneath you isn't F-Nero or something and decides to chase your mid-screen airtech too late because you mixed up where you teched and how you moved after it, they usually will eat a jumpin and full combo. However, if you're in the corner, this usually makes the airteching game far easier for the opponent, though if they guess wrong if you have air movement left and commit too hard, you will either get away or land a hit.
Overall, that's why unless you're facing specific characters or are in a shitty situation, airteching is pretty damn good in MB--- especially mid-screen.
As for Ryougi's tech, if the opponent is trying to chase your techs in the air you are betting far too much on them being at the right spot at the right time hitting a button. This is why you don't see it often in these situations. In fact, the only place where her airtech counter is even viable imo is when you teched out of an aircombo that was messed up. Otherwise, her airdash and good air normals lead to far safer decisions.
P.S.: To emphasize, I know the shitty situations where you can easily be guard break'd, as well as what characters you don't want to easily airtech against, like Nero; I'm giving a general gist overall. Though to be fair I'm probably underplaying the airtech situation game in the corner, since that's actually fairly good.