Pikachus bair is bad. If he short hops it you can mash up on both sticks , get out of it, land and punish his terrible landing lag. If he full hops or double jumps it, you can mash both sticks down and get out of it. You might not get a guaranteed punish, but now pikachu is above you and you are at positional advantage.
It may have a low sdi effect but it hits so many times for so long, that it doesnt matter. Even people with average reaction times and suboptimal sdi methods like me can do it.
Pika fair is very good because you can land with it with very little lag. If that lands it combos into 90% of the characters moveset. Including kill moves like nair, up smash and up tilt. The latter 2 obv. Being folloed up by thunder. WHICH IS LIKE ONE OF THE BEST MOVES IN THE GAME. Zero risk unpunishable kill move attempt? Yes please.
Bairs are generally great in this game. Which is weird because in any other fighting game, someone being behind you is the WORST position to be in.
As a point of reference. Sonics bair, in the context of all the bairs in this game is really average. But its still one of his top 3 moves for him as a character.