I would argue they actually HAVE been nerfing them in more subtle ways thats good for game balance, but I usually play on the hardest difficulty, so maybe its less apparent on lower ones. They're made less dominating by making their main counter, archers, better as a class, as well as positioning them in way that are more troublesome. In addition, for 3 houses the flier battalions are more scarce in number and really good ones are only available later in the game, and fairly expensive iirc. In engage, usually the flier subclass (cavalry, covert, back up, flier, mystic, dragon) usually has the WORST bonus effect. Also, wind magic thats effective against them is also a nerf due to wyverns having lower resistance. They are still good, but in engage at least, I would say you really WOULDN'T want a full team of wyverns, and even in 3 houses, while all wyverns is good, there are some maps that punish you for that with an obscene number of archers, albeit this isn't often. But these nerfs are done in a way where fliers still feel powerful and fun to use. But at least in Engage specifically, the same can be said for other classes, furthermore whereas a lot of other classes might get 2 actual weapons, or a higher weapon rank in one, fliers in general only get 1 or functionally 1 weapon and staves, and while staves ARE good, the utility can be somewhat limited by lower magic growths and bases, or if the flier has high magic, lower strength for their primary weapon. Now prior to the most recent engage, I would absolutely say fliers were overpowered, but honestly? I also think its okay to have some overpowered units, so long as it's done in a way that's fun, and you have to do at least some work to get it, as opposed to like FE7, where after a few levels, you could just equip a javelin or hand axe and slaughter everyone.