In XY Braviary isn't NU. It's in Limbo, which means it isn't decided yet for now.
However in Gen 5 it WAS In NU. Why?
First off it's typing being Normal/Flying is rather tame. It's not a great typing, with both STABs being walled by Rock and Steel types. As such, Braviary has very little coverage to take on Steel and Rock types. Braviary's low base 80 speed kind of offset's it's high base 123 Attack, making it less fearsom than it would be - though that can be remedied by using Choice Scarf.
Anyway if you look at the Smogon Analysis for Gen 5 you can see there are analysis for RU and UU as well - so Braviary is useable in the higher tiers as well. It's not OU Material since the mighty Staraptor exists in BL which is essentially OU, and it has a much better base 100 speed.
Anyway so yeah, the main problem with Braviary is it's lackluster speed, which is kind of low compared to even some of the top threats in NU - move it into RU or higher, and it would be worse. I