As with any organization that large, the quality does vary from branch to branch. However, Barra is a bit of a special case. They were the largest of the pioneers of sport BJJ. They have MANY world champions. Many of those world champions knew they could make some money opening their own schools in places other than Brazil when MMA and BJJ exploded, and so they did.
Because of that, many of the Barra branches are taught by guys that are former world champions, former head instructors at Barra, etc. The thing is, training in that environment in Brazil with so many guys of that caliber means you essentially HAD to be a badass to achieve a black belt. So, when all those black belts went out to start their own schools, the quality level stayed similarly high.
The other thing Barra does are lots of visits by black belts to affiliate branches for seminars and such. That tends to help keep an eye on the affiliates so they aren't too ridiculously bad. Are there some bad Barra affiliates? Probably, but I've never seen one. If you go to a Gracie Barra school in a major city, it will be an official school taught by a Barra Black belt. He will almost certainly be a badass, and chances are good he scored a mundials championship at some point. If you go to a school in west bumblefuck north dakota, well what did you expect? Nothing there is going to be awesome, not the weather, not the women, and probably not the BJJ instruction.