Every situation is unique. This is my 140lb English mastiff/South African mastiff Bubba sporting his smile face when I come home.
He is a great deterrent & would be far from easily defeatable for 99% of the population but still vulnerable to one little guy with a gun.
Having 3 or 4 of these kind of dogs around would be a lot of fun
One day I'll have a small pack of dogs that roam a perimeter fenced in ring, and dogs in general definitely function as one layer of many for residential security. :) I certainly wouldn't want to mess with Bubba! LOL