They are gentle giants but there is a flip side too.
I own a male Bullmastiff mix and he is very calm,cool and collective but his guardian instinct is 100% on point,he is very good with people once he has met them,before then they are a stranger and are treated as such.
I've read up quite a bit on the bullmastiff and the females are said to be non tolerant of the same sex sometimes,my dog is great with 99% of dogs but there have been rare exceptions (normally from dogs who seem to want trouble so he will give it back) its because of his exceptions he is never offleash,your OP asks about the pit bull and comparisons to the bullmastiff,lets put it this way ive had the unfortunate experience of seeing my mastiff fight a dog that attacked him offleash(a pit bull type dog),their power when they want to do damage surpasses that of pit bulls IMO,its because of this they should only be owned by people who take the time to train and socialize them.
One of my Staffordshire bull terriers had serious DA problems and although he and the mastiff were very close i knew if they ever did fight seriously,the mastiff would wipe the floor with him,way too powerful.
That being said..there is no love better than mastiff love,even the drool becomes something you don't care about because a mastiff is likely the best friend you'll ever have