As someone who has worked in one of the most professional of environments - the army - with guard and perimeter dogs - mostly Mastiff derivatives - I can confidently say that there is no such thing as a safe guard or protection dog in the yard of the average household, without a competent handler to give directions.
Your average dog, whether manstopper or not, does not distinguish between an axe murderer, or the paperboy, when left to its own devices. If one employs such questionable tactics as relying on a so-called guard or protection dog, in a society such as ours, then this would be akin to setting a deadly booby trap in one's yard or home, which eventually will go off in one or both ways; towards oneself, or towards an unsuspecting, innocent victim.
The chances of an axe murderer threatening one's home or life is a highly remote possibility, however, the chances of the paper boy getting attacked and seriously hurt or killed are better than average... and that is what usually happens.
Unless in the company of a highly trained handler, guard dogs are of no use. When alone they are easily surpassed or eliminated by simple methods, and that being said, all there is left is the imminent danger to society on or outside their turf.
To the naive or arrogant, thinking of getting themselves a Mastiff or Bullterrier derivative to do the dirty work for them - and I am excluding the criminal audience beyond hope here - I have to say this: Do not get fooled by wrong advice from suspect sources, the questionable breeder, or the irresponsible guard dog industry. Chances are that it will blow up in your face, literally, and you will be left without one... or worse, as it has been so many times the case lately, there is a high probabilty that an innocent victim will suffer the consequences... a victim maybe as close as your toddler.