A staffy is going to weigh about 12-15kg if exercised and fed properly, an XL bully is double/triple that at least, so that is 64kg of pure muscle or ten stone of pure killing machine if you look at it the wrong way one day, when its inbreeding causes a well documented and known about malfunction !
Humans are rubbish against large predators in general because we haven't needed to be, so we have lots of lovely arteries close to the surface which powerful, teeth filled jaws will puncture easily, or even rip limbs clean off, as has occurred in at least two cases, I have seen in the news.
Looking at my saggy, baggy, elderly, largely toothless labrador cross and wondering why these people don't choose a more benign canine companion ?