well, the good points about braviary have already been outlined. but the issue with braviary is that a lot of the power or speed backing it up comes from being choiced which is really irritating for a stealth rock weak pokémon when the user predicts wrong. and admittedly, being choice locked into superpower can suck. for sub + bu, it doesn't really have that immediate offensive prowess so it can be pressured by offense. it also kind of suffers from having to use one attack. brave bird does hurt electric types even if it's resisted, but there are still a fair amount of bulky ones that comfortably take one at +1 and get off at least two attacks.
now, these cons are easily outweighed by the benefits. even though the user needs to predict with a choiced braviary, so does the opponent. a lot of the rock type checks lack recovery, so they can be worn down or can be taken out with a well-timed superpower. even though stealth rock limits the hit-and-run nature of braviary, it's bound to get at least a kill in the right hands. sub + bu practically shits on almost anything using a weak/uninvested attack which is pretty common with all the regenerator cores or whatnot. it's not even limited to this too, which makes braviary so good.
i wouldn't exactly call it versatility (even though i did say that once). it's just that the choiced and sub + bu sets play extremely differently that you may just need that extra check.