I mostly use NAir after UAir. I mean, is there ever a time where I'd want to use BAir instead?
I use bair after uair because it has a wider hitbox. That said, I'm trying to use nair more [I for some reason get a lot of sourspot nairs after uair, something I need to grind in training or something to fix].
[Also, if my calculations are correct, bair does marginally more damage... nair does 8.5%, bair does .8*7=5.6+3 = 8.6%, although if the move stales as it is used than this is inaccurate.]
I don't know if this question can answer, but I'll go ahead and say it.
If any of you Pikachu players have good experience against Samus, not Zero suit Samus, then please let me know and I'll do my best to improve. I just faced a really good Samus player and had fun, but I had a lot of trouble against Samus. Approaching her was the biggest problem because I keep getting wall out by her aerials, charge shots, and missiles. I tried being aggressive against her and put out pressure, but it didn't work because I keep having a difficult time reading her aerials which is hard to punish. I got in a few times, but I end up getting baited a couple of times and get charge shot at. Jiggling her is difficult thanks to her down-b. Well, that's all I have so I hope that your advice helps so I can get better with the MU as Pikachu.
We can walk/run under super missiles [test this to see if it's both or just one of them] and can definitely crawl under them.. Her SH fair is laggy, and her grab is not that fast so you can shield more in this MU than usual.
Samus should have issues landing... while down+B is good, if they use it a lot, just wait it out and punish it with fair or something. I could possibly offer more advice later... but one important thing is that you will occasionally have to shield projectiles in this MU, but when you do, occasionally consider doing Quick Attack OoS towards her to immediately close the gap. Yes, she can read this option, but if you do it rarely, it will let you start to quickly apply pressure.
Uthrow is better because it's faster making it harder to react to with DI and it has less recovery which makes it less likely they'll dodge a guarantee thunder follow up.
And bair can be used the same way as nair as a follow up after uairs. They both have similar damage, knockback, and speed. There isn't much of a difference between them as a finsher except you can't use bair near the ground because you won't get all the hits. I guess if you don't want to stale one then use the other. But which move is better to keep fresh?
I use bair a LOT [I think it's something I should change], but I will state that fresh nair offstage seems good for getting KOs while edgeguarding. I know bair has more KB (or it looks like it), but people can SDI bair to where they'll be stage-spiked [and they can thus be prepared to tech it or else play the 50-50 of tech vs no tech] or even just launched back onstage... while you (mostly) can prevent this by fading back with them in your bair, you're then taking them farther away from the blastzone, so I don't know if it actually KOs earlier than nair in that case.
[I also edgeguard with dair a lot... do bair/nair really have more KB than offstage dair? I always thought dair and nair were about equal...]
EDIT:
Stuff about b-reverse thunder said:
It theoretically has value in a teams situation where your teammate just go star KO'd... stall offstage with this for a while, since most people will have no idea how to punish it, even with two people, until your teammate gets back. It could also be used to end a timeout, but I don't know if it's worth learning it for either situation.