Those breeds are banned in many states/cities because of how dangerous they are, and then you HAVE to have insurance for them, which costs almost as much as a small studio apartment does, and then the food and then no one will come near you once they find out what breed it is, and people will call the police on you saying yoru dog is trying to attack them even though it isnt so they can have it taken away. ;-;
my friend has a rottweiler and he's one of the sweetest dogs i've met. none of those dog breeds are necessarily dangerous, they're just used most often in dogfighting rings and by people that use dogs to hurt other people because they're bigger, stronger breeds and that's given them a bad image. they all have the same temperament as any other breed of dog unless they're treated to act otherwise. in terms of how often they bite, smaller dogs are actually more dangerous than these bigger dogs. there's no such thing as a bad dog, only bad owners; it all comes down to how you treat them.