Cesar Millan who hosts the show Dog Whisperer on the National Geographic Channel refers to those big powerful dogs, Cane Corsos, Presa Canarios, Bull Mastiffs, “Pit Bulls”, Rottweilers, and German Shepherds, as "gladiator breeds". These are all very high energy, powerful dogs with strong personalities. They need owners who are experienced and know how to establish themselves as pack leader. If inexperienced owners take on these breeds and don't properly train them, bad things can happen. I mean, how many pit, or pit mix owners have a breaking stick and know how to use it?
I think it is foolish to not acknowledge the history of a breed. Dogs that have been bred to fight other dogs, or to be protective, ARE going to have a greater tendency to display aggressiveness. I've read opinions from experienced Pit Bull owners that Pits should never be left unattended with other dogs, because even dogs who've never show aggression can "go hot". This comes from people who love and admire these dogs, but recognize that they have a history that can predispose them to some behaviors.
I don't think there are "bad breeds", but I do think there are breeds which require experienced owners, because if something does go wrong with such a powerful dog, it's going to be VERY bad.