No, they’re no more dangerous than any other dog is. Despite their size and intimidating presence, Bullmastiffs are actually gentle giants who are more suited to life indoors spent lazing around on the couch than one in which they’re left to roam the streets or are chained up in a yard. But, like any other dog, a Bullmastiff can become aggressive if he’s treated badly and neglected, and because of their size and build, if a Bullmastiff does become aggressive due to maltreatment, he could be extremely dangerous.