Yes there are spoiled, dangerous horses originating from "ANY" type of training method. I don't care what name you attach to it, BAD TRAINING, IS BAD TRAINING.
I have the unique opportunity to travel around and see many different horses on a daily basis because I'm a barefoot trimmer. I work on horses ranging from week old foals to 38 year old toothless seniors. Thoroughbreds to Fjords. Gypsy Vanner to Quarter Horse. Arabian to POA. Paso Fino to Percheron. This is my full time job. I work with horses every single day.
So far, I can count about 7 horses that I would say were outright dangerous, not safe to trim or be around. One was labeled a "farrier killer," one has to be tied up short to keep her teeth off your face. One was so wild he kicked my tools all over the barn from one end to the other, knocked down both handlers trying to control him, and then bolted through a gate and took it off the hinges. I never even touched him! That all happened within the first 2 minutes of him walking into the barn aisle. One was a yearling who had already gone through 3 trainers and had to be worked to a lathered sweat before anybody could touch his feet and he still kicked out violently with both hinds. One was a 30 year old gelding that just had "moods" and "bad days" and if he was being pissy and crabby, he was all teeth and heels and nobody could touch him. There were times I had to just skip him because they said he was in a "bad mood." Owner said he's been that way his entire life! One hand to be drugged and twitched to be led from the pasture to the barn. One mare I never even met but a new client set me up to trim her other horses, then told me about this mare she had just given away, and I should be glad because it was a total fiasco to trim her. Another gelding I never met - the owner told me about him. He attacked a farrier with his teeth and threw him over a pipe rail gate. She had him put down.....And another I didn't include in the 7 was a young Appy gelding who wasn't dangerous, but he was difficult. He could just NEVER get it. He would fall down, stomp his feet, snatch his legs. I trimmed him for about 3 years and that horse never got better. Every time I was out to see him, it was circus to get him done. I do work with difficult or untrained horses a lot and after about 2 trims or so, they are usually doing really great. But not that guy! Boy he was tough.
You know what EVERY one of them had in common? Purebred Appaloosa. Every single one of them! It's almost become a joke - if somebody tells me about a major behavior or emotional problem with a horse, I almost always ask, "An Appy, per chance?"
But you know - I stll don't think there is one thing wrong with the Appy breed as a whole. My mother raised purebred Apps before I was born. I am guessing they get into trouble because people select for specific traits and colors, and so you might be more likely to breed bad temperaments just to get specific traits and colors.
Appaloosa horses are VERY intelligent! I have found them to be more like a mule almost, in that they tend to really think, and they need really good training in most cases. So I believe I tend to see and hear about so many "Appys gone wild" because of bad genetics, and bad training. But it's no fault of the breed!
But the point of this is to say, you can draw any parallell you want on any topic you want, but it doesn't mean it's accurate. I bet if polled all of you, you might say you have NEVER in your life encountered a poorly behaved Appaloosa horse. Every one you've ever been around has been awesome! So there is no way I can let my experience with Appy horses in my little corner of the world taint my view of Appy horses the world over! That would be absolutely silly. Even if I were to encounter 50 "bad" Appys, that is 50 out of how many hundreds of thousands in the world?! It would be absolutely silly and uneducated for me to declare that the breed is bad based on my own little sampling of horses.
I trim some really awesome fantastic Parelli trained horses, and I trim some spoiled brats that are owned by people who "do Parelli." It all depends on the human initiating the training - NOT te method used, or the breed, or age of the horse, or anything else. Anything that is "wrong" with a horse can be pinned back on the human doing the training.
Is it easy for people to just buy some DVDs and call themself a horse trainer - thereby ruining horses? Sure! Same way it's easy for people to buy a pretty Appy and then ruin it because they just don't have the sensitivity or knowledge to train such an intelligent, thinking type of breed. But I don't believe the answer is to destroy the guy who produces the DVD, or eradicate the offending breed of horse. People just need to get smarter about the way they train and handle horses
I should add of those 7 difficult horses I described, only 1 was owned by a woman I would say "does Parelli" and her other horses were completely fine.