the real danger of nair is that it can be crouch canceled by sheik and other characters that like to stay on the ground
nair is generally a spacing tool that is used to to keep away the likes of puff and to continue or finish juggle combos or tech reads. just because your opponent can potentially sdi out of the 2nd hit doesn't mean that when the situation arises to use nair that you should opt for another move.
also, if you assume that your opponent is so good that they will sdi your nair every time, but still be put in situations where you can edge guard them with nair, then I'm noticing a big skill gap in that kind of player. Naturally peach and puff will come in high, but usually too high for you to punish. others might go high as a mix up, but generally people are going to sweet spot the ledge. Regardless, someone with the skill to consistently read and SDI your nairs will probably not be put in such bad situations to begin with. Watch pro players; even at the top level there are players that miss the SDI for fox's up air. when people start sdi-ing marth's nair then this thread will matter. Getting crouch canceled is much worse and a bigger reason not to use nair (against characters that are prone to CC).