The first set of problems comes from the breeding shed. Horses from World Grand Championship lines are bred to pace, not walk. So if you want a "natural walking horse" the last place you want to go is blood that is heavy with Big Lick ribbon winners. Remember that breeding counts in horses.
The next set comes from putting a horse into Big Lick training so that they can compete as two year olds. This means in most cases they've been in "colt packages" since the age of about 12 months, go under saddle at 18 months in adult packages, and are only worked under saddle (very little basic ground work). The last part of a horse to mature is the back. This training regime will badly stress the back.
But not just the back. I've seen a lot of ex-Big Lick horses that have problems in the stiffles, hips, shoulders, hocks, and pasterns. Look at a Big Lick horse standing square (not in the exagerated "conformation" photos so often seen which are, by nature, a fine way of concealing conformational defects) and imagine the "force lines" which will be generated by movement.
The more lateral the gait the more hollow the back. A Big Lick horse is bred to pace and that pacing is "squared up" by the package. At least according to Big Lick theory. In reality they do not then become a "square moving" horse. The back of the Big Lick horse will remain ventroflexed. The real purpose of the package is to increase front end action (erroniously called "animation" in most Big Lick literature). This will substantially increase the force levels mentioned above.
Frankly these horses are a classic example of the "pig in a poke." Every now and then one comes through the training without injury, but the vast majority will
have physical consequences from the mileau of Big Lick breeding/training. This means the buyer must have a lot more than just a good PPE. They have to have a clear view of what they want from a horse and what the horse in front of them offers. Do not buy one of these horses because you want to rescue it. Not unless you have deep pockets (for the vet bills) and enjoy disapointment from continous lameness issues.
Get Dr. Deb Bennett's series of books on conformation analysis (there's three of them for about $13/ea. at Amazon). Learn the basics of good equine conformation, and movement, and apply them to the horse you're looking at.
There's an old saying that goes Prior Planning Prevents P**s Poor Performance
." Some folks call it the Six P Principle
The planning, here, is first the education of the human. Learn and pactice applying the principles of conformational analysis. This will naturally lead into the ability to analyze gait and movement. You'll also learn to read temperment. Then find out the needs of the discipline(s) in which you are interested. Then go find a properly conformed and moving horse with a temperment that fits those activities. This is not a "weekend project." It will take some time. But the payoff is a sound, sane horse that will work for you for years to come.