Just on mastiffs in general:
For 5 years,I had an English mastiff live with me for 3-4 months of the year while his owner worked in another state over the winter.For most of that time I would say that other than covering everything in black hair and slobber,snoring like a idling bulldozer and laying farts of mass destruction he was one of the best dogs I'd ever met.
However,one day he decided that he no longer liked someone who he had been around in my home probably a dozen times out of the clear blue and made 2 honest attempts to bite her.Granted,I'm a fairly big dude but 195lbs of dog is a hell of a lot to restrain when they have something else on their mind.I do not have the absolute slightest clue as to what may have triggered it but I had to tell his owner that I could no longer accept that responsibility as people>dogs in my view,not to mention that my home ins. specifically does excludes mastiffs and had he managed to make contact other than sleeve of the coat,it would have been both of our asses.
I'm not trying to scare you away from the idea of owning a mastiff,just want to share my tale of a dog doing something entirely out of character and that a big dog is a big responsibility.