The kill move Bair is pretty hot offstage. And as an Uthrow followup. Plus it autocancels on short hop, so it's safer on shield in that regard. I mean, did Bowsers really fall on people's shields with Bair in Brawl? I will agree though that the loss in active frames is pretty noteworthy. I can't imagine anybody cared about moves like that in previous games, but for smash 4 where you need attacks to cover one or 2 frame vulnerability scenarios at the ledge, that's a big deal. Nair can guarantee a hit for those scenarios, but it doesn't send them back out there or kill in its own right.
But overall, like many of Bowser's moves in the transition between games, I'd say Bair is a net gain. Most characters would kill to have an aerial with that much damage and knockback and only 9 frames of startup.