Pit bulls are wonderful dogs. We have a pit bull mix who is so incredibly chill and loving, and we've received many compliments on his behavior. He gets along with all the people and dogs we sit for, even the difficult ones. Yet, even he can get protective of us and the house. It's only happened with one person, but when our dog met him, he immediately started barking (and he does NOT generally bark). Later, when this same person came to check in on the dogs for us one day, our dog wouldn't let him in the house to take care of him and began barking, growling, and even charging at him until he left the house. Between these two occurrences, he was totally fine and appeared comfortable since we were around. Dogs are extremely good at reading intention and your comfort level, and I think our dog picked up on the fact that a) my husband didn't like this visitor initially, b) the visitor was not very familiar with dogs, and c) this person was a very big, muscly dude.
Also, remember that just having dogs, especially big dogs, and certainly a breed with such a huge stigma, can deter anyone from attacking you or entering your home (if they're aware you have dogs). I've even heard of people posting "Beware of Dog" signs to deter burglars and such.
There's no way to guarantee that your dogs will protect you in situations in which you're in danger unless you get them trained to be protection dogs. Aside from that, you can always teach them a "speak" command so they can bark on cue and sound intimidating. That's all I got, hope it's helpful.