I spent better than 30 years re-training spoiled horses. Many were headed for slaughter and I was their last hope for survival. For most of those years, all the spoiled ones were brought by customers. It seems that when you straighten out so many outlaws that had put people in the hospital (some nearly killed by horses and several crippled for life) , the word spreads and you attract every other dink that has been badly spoiled. The last few years that I re-trained them, I quit taking in outside horses and just went to sales picking all of the spoiled horses that were well-bred, good looking, sound and old enough to ride hard. Most were 5 years or older. Some were flippers, many only turned one way and reared or threatened to rear, some were led through and only a story was told or I talked to the owner before the sale.
I can tell you that almost without exception, they became very useful horses and some became really nice show horses. One shown in our website went on to win 6 AQHA World Championships and over 30 AQHA World Show 'Top Tens'. He was an outlaw when I brought him home. Some went on be big-time hunters, jumpers and dressage horses.
I very seldom ever hit any of these spoiled horses and cannot remember using a whip on a single one. BUT, you can bet I dominated them and made them all say "Yes Mam!" and back up when I indicated to them that I wanted them to back up. Their ONLY reward was that I took all pressure off and left them alone when they did the right thing. They all became completely relaxed and happy to do what they were asked to do.
Sometimes the process was not pretty. [I always said it was much like making sausage or bologna. You only wanted the end result and did not want to watch the process of making it.]
When we have had someone working for us, we always stressed that there were two completely different training techniques -- one for green un-trained horses and a completely different one for spoiled horses. Training green horses means that you are trying to teach them to do what you want. There is NEVER any reason for it to be 'rough', much less abusive. Re-training badly spoiled horses, particularly very aggressive horses, can sometimes get pretty rough. You have to have a horse submit to get them to completely 'give up' a bad behavior. Otherwise, they just go back to it when a different person tries to handle or ride them.