Nair doesn't protect underneath you or above you. So if you were playing against a faster character odds are you will get punished for fulljump nairing if they are close enough. Most characters can grab you from the air before you can reach optimal wavelanding range. So it really depends on the circumstances (spacing, are they approaching, etc.).
I've gotten people with the sh fair to waveland though. It's pretty effective if you went to combo with a sh double fair and they jumped out before you hit them. Normally they will come down on you if you continue with the double fair anyway, so wavelanding back can net a fsmash or grab.
I've been thinking about this, you shouldn't ask if it's a good mindgame tactic. If it works for you against someone that's all that matters. No mindgame is always going to work against good players or bad players. People claiming whether it's bad or good will only hinder your development. Use it until it works, tweak it if it doesn't. Then change it if you find something better. That's the best way to increase your ability to create mindgames (hate the term) on the fly.
