Geez, this kinda topic can get as heated as politicos...lol
Here's my 3 cents:
It is all about the dog, they were bred to be aggressive AND with the strongest jaws to inflict the most damage possible.
It is all about the owners, too. You got the good owners who train or get the terriers trained properly, thus better behaved dogs, maybe. Then ya' got the asswholes, that train 'em to eat cats AND be aggressive towards people, too aka, guarding.
Then ya' got the dog my family had...Amer. Stradfordshire, great family dog. We had 3 young kids and our house was Kid Central Station for the neighbors, too. From birth I trained this dog well. One of the things I learned, was how to train away some especially dangerous traits.
I did/do specific things that helped. When a puppy is eating...take the food away, only when he stops bad behavior does he get it back, everyone in our family would take turns doing this...all during their growing and later. They would get mad, like most dogs, but, they were taught unless they let you and others be "Dominent" they would not get the food back. This same kind of passive training works in other areas where dogs have trigger points and may typically attack.
Another thing you can do is hold your puppy down, don't hurt him, make him be still. He will wiggle and whine, then seeming give in. He's not ready yet...after he stays still a few minutes he'll get his 2nd wind, hang on -- now he's gonna get mad, maybe try to bite, too. Lay on him carefully until he finally submits, he'll let out his air and relax...you've won. Pet up a storm of "Good-boys"
Some may not like this, but it's easier to deal with young dogs than grown big ones. They need these passive reinforcements as reminders from time to time.
Back to my AS Terrier., despite everything she attacked ...for no apparant reason a young Rough Collie we also had had for a long time. She had her by the throat and would not let go...it was like she was in a trance...I ended up hitting my pet with brooms, boards anything I could find, nothing worked. A neighbor went for a gun...I tried water hoses, soaking her w/cold water. I felt mean and cruel the way I was beating that dog, that my 2 year old daughter could take food away from ...without a grumble.. I hit her with a 2 x 4 -- NOTHING. I finally got a smooth dull iron rod and got it wedged into her jaws...and twisted out a jaw tooth !
She let go...I thought she was going after me or others now...or back to finish off our expensive "Lassie" type dog. NOPE, she felt sorry for her victim lying on the ground bleeding and panting, she started licking and cleaning her up. We were have a party, so all the kids and people evacuated, thinking "Mad Dog." NOPE, I even petted her later, checked for wounds from me, there were none, except the tooth.
Our young Collie, who was much larger than our sweet Strad, had multiple deep bites all over, most of the hair was gone around the neck. It took a while , but she survived.
The moral of the story is: Figure it out yourself.
Keep in mind, the entire time I was trying to stop this dog attack, I saw my own daughter neck flopping around in those looked jaws, or a neighbors kid. Yes, it's true Pit Bulls and kin don't bite the most people by a long shot, but, because people have bred these dogs this way, they will cause much more serious damage, because they won't stop. I've been bit by 3 dogs... German Shepherd, Doberman and Dobie/Shepherd mix. THEY BITE AND RUN...that's livable.
Recently, a Amer.Pit Bull, don't know his or his owners history, he broke out and started jumping fences -- he killed about 25 - 30 dogs in their own yards, a few cats, too, he chomped on 2 - 3 people (multiple bites) trying to save their dogs.
The dog dog goes home and bites the owners 3 y/o kid about 50 - 60 times, he has the boys head and jaw in his jaws. The man beats his dog like I did mine, NOTHING, he get a .22 cal. rifle and carefully shoots the dogs head several times, the bullets bounce off the skull. Finally, a shot thru the eye, enters the brain and kills the dog. The boy survives, charges are filed.
The family is interviewed, "Well, ar ole' dawg, never dun nuthin' lak thet ba'fore...He wuz a good dawg, now th' Sherriffs sayin' thar gonna kill ole' Red...daid." Everytime it happens -- that's what they say.
This happened in Texas within the last year, I do not have the copy, it was on TV, too. I may not be factually accurate on every little detail, but, it did happen.
I had my dog put down, I loved her, she was the family favorite, never hinted of trouble. IMHO, her genetics were triggered one day, she had an ability inbred and she simply acted on it. She was trained NOT to be aggressive to people or our other pet animals.
But...I will never again take a chance with certain breeds, it's a gamble. If you lose (rare) you lose BIG.