I haven't really found this to be true. A lot of people complain about pit bulls or rottweilers or dobermans, but I've run across plenty of these dogs that well trained. I've also run across some pit bulls who were trained to be nasty.
Like a neighbor of my grandmothers had a pit bull and would train it by having it jump at a cloth on a rope, while beating it down and kicking it. They had the rope swinging from the roof and the dog would play for hours, jumping in the air and biting the cloth, and hanging there about 4 feet in the air and not letting go.
I actually played with that poor dog when it got loose on several occasions, and it never bit me. In fact, I experimented on it a bit by seeing how good its grip was, by playing fetch with a chew toy and then trying to pull it away. Pit bulls have incredible jaw strength.