Many years ago I read an article about Bull Mastiffs (which St Bernard's are bred from. The article said NEVER to hit the dog. My thought at the time was that I sure wouldn't want to hit a dog that big. But the reason for the advice was not to protect the person striking the dog...but rather the dog itself. Mastiffs are so gentle that hitting them breaks their spirit.
Most of the giant breeds have all the aggression bred out of them. I just don't understand this story.
We have some close friends who have bull mastiffs, and my experience with them is that you are spot on.