I have a black lab that is the dearest dog on earth. But I have seen her "Wall O' Teeth" act when confronted with the an intruder and I wouldn't want to be on the receiving side of it, no way, no how.
I had a half Lab half Shepperd mix years ago. She was great. Very firendly to my firends, very nasty to strangers until intoroduced. Later on in her life, the Shepperd side got the best of her (Hip Displasia(?)) and a little arthritis, and she would just lie in the breakfast room under the window. When a car pulled up she'd prick up an ear. If it was one of my friends she'd go back to sleep. If it was an unknown vehicle she'd get right up like there was not physical pain or discomfort and bark and growl at the stranger like always.
I miss that girl. Good dog.