EDIT: @
Trifroze
I would agree but as I said below, Nario was pulling this **** off on PIkachu.
I really don't know any more.
The Up B usually fails when the Uair knocks them too high and they get hit by the second half of the Up B, so they pop out before the final kick.
However in my experience as long as you start Up B right next to them and don't move to the left or right, they can DI however they want but they're gonna get hit with the whole move.
I used to think ZSS' Uair Up B ladder required very specific percent ranges but Nairo pulls it off almost every time he goes for it, even on tiny characters like Pikachu. So maybe it just requires, y'know, some practice. Or rather, even though the percent range is still specific, it's far from tiny.
Considering Nairo's consistency, I don't think "they can fall out of Up B" is much of a defense any more.