If it's a good trainer, it won't fail. I had adopted my dobie when he was about 3yo and trust me, he was a demon, he was scared that he or me would get murdered so he would attack like anyone and anything he was comming across during his walk (ofcourse he was wearing a muzzle). I came across a couple of "positive" trainers but instead of fixin the doggo they kind of made him worse...luckily I found a well known dog show/guardian trainer who FIXED him. It's been 7 years now, havent used the muzzle once, hasnt attacked anyone or anything, the dog is happy to go for walks without thinking he prolly die. My story is about how a trainer differs from a pro trainer, except he does better job on the dog, the most important part is to train you to handle the dog, dogs are simple thinkers, if you understand how their brains work then he won't cause any trouble at all.