Large intimidating dogs
Berners, as they’re lovingly called, are very gentle, considerate dogs.
The Bullmastiff is a large, very imposing guardian breed.
Both guard dogs and watchdogs bark loudly to alert their owners of an intruder's presence and to scare away the intruder.
The watch dog's function ends here; a guard dog is capable of attacking, or restraining the intruder.
With careful training and patience, though, those problems can be eradicated and you can have peace of mind concerning your choice of pet.