Reverse nair is actually not completely safe (your back is open to damage) but it's still a large hitbox with a long duration making it a good option in many situations.
One of the keys to playing jigglypuff well is to
not be vulnerable when you are landing. That means
not landing in lag frames (even an l-cancelled aerial will put you in danger/lag frames) and staying in
a difficult to approach state (hitbox out or capable of being out) for as long as possible. Nair is used when you are low on jumps and need to land because the hitbox lasts for a long time and autocancels one frame after the attack ends (aka HOLYCRAPAWESOME).
A non-reverse nair is actually preferred, but sometimes you want the hitbox out fast and dont want to waste the time/jumps to turn around. Once you get used to the timing of landing with autocancelled nair, it becomes a good option to avoid run-up grabs. Just keep in mind that puff falls slow and nair will still lose to most vertical/angled attacks so it's not really safe to use next to a shield or from directly above.
