I wouldn't be too worried about an Appaloosa. They have a tendency to be a little stubborn, but they are pretty hardy horses. They are just like any other horse and they have good and bad days just like any other horse. If the horse is trained properly, free of vices and healthy, the horse should be just fine. I wouldn't worry about them exploding, heck any horse can do that for no reason whatsoever. Sometimes they just need good guidance and you have to be firm about what you want them to do. Just like any other horse. I would definately make sure that the horse has good ground training and respects you on the ground, because if he doesn't, he won't respect you in the saddle.