Nair for an edge guard? not a chance. I think that peepz are placing too much emphasis on this move. Let me remind you all that nair is a low priority and low range move. You will get blasted with any up b move. I don't think that it would be a good idea. D-smash all the way!
nair as an edge guard?
a definate yes.
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if your opponent is well below the stage and u have the time, fall down and nair him.
characters such as fox needs a bit of time before they activate their UpB.
the nair, in this scenario, has a good chance of stage spiking them or knocking them back too far away to recover with their upB.
again, in the situation where your opponent is below the stage recovering, some characters take time to recover and their recoveries dont barely pose any thread when they are below you. in these situations, nair is also a good alternative to bair and uair. since, bair and nair doesnt hit directly below you and dair is too hard to aim.
examples for this situation include ... fox, link, pichu, pikachu, cfalcon, ganon.
take note that if you plan of stage spiking your target, you are likely falling and nairing BEHIND him... meaning his upB is unlikely to hit you.
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in the air, lots of characters are vulnerable, because using aerials can drag them down too far below the stage. for those characters, just about any attack with a knockback is a good idea
the the last shock of nair is convenient because it either projects your target left or right with set angles and projectility. The nair not back doesnt throw targets in random direction.
on top of that, the timing isnt hard to catch. nair last for roughly 1 second.
usually, run out to your opponent with a full hop nair and use fast fall as well as DI to aim your last shock.
practically, it isnt hard to aim at all.
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knowing these, nair can serves as a good alternative to for bair/uair depending on situations, your direction, positioning, and etc.
sry that my response is long... hope it doesnt make it less appealing to read