>>I think Bull Mastiffs are listed among the more dangerous dogs
I don’t believe they are. Not in the top 10 here, for instance: http://dangerousdogs.net/
Good friends have owned a series of Bull Mastiffs over 3 decades plus, usually two at a time. They’ve been sweet dogs. Their most dangerous aspect in my experience is the drool. It can be massive.
You may well be right, I’ve been bitten by a German Shepherd on the wrist and I take care of dogs, there are different factors including sometimes bad owners.