Interesting...I had never heard that about them, but I have plenty of thoughts on the matter.
My step mom used to have an appendix QH - a dash for cash boy. He sent her to the hospital 3 times.from bucking - Broken wrist, dislocated shoulder, cut open leg artery - causing her to be sent in a medical helicopter. That one finally made her sell him.
My sister sent her appendix QH to us when she could not sell him (also a dash for cash boy). He had a nice enough personality, but there was something else going on with him. He spooked a lot, bucked a lot, pulled back when tied, would blow out of the horse trailer, Overall, very explosive.
I think there are pleny of nice appendix QHs out there, but the problem is that they are so big usually that when they misbehave like a 15H horse would, they have more power, are more explosive, and just more dangerous.
They are often off the track giving them a whole set of problems both physical and mental.
I have seen many that are physically unsound (constantly becoming lame due to one part of their body or another). They need corrective shoes, chiropractors, vet visits, time off, special pads, special saddles, etc. ( this is of course true of other horses as well and does not apply to all appendix, but the numbers seem to be high).
I also think that they are incredible athletes when everything is working for them mentally and physically, so this draws people to getting them for high impact/high stress disciplines - finishing the cycle with a stressed out, dangerously explosive, and often lame horse.
Both of the horses I described above were barrel horses and the second one was off the track. These kind of horses are popular with barrel racers because of their speed and agility, but again, I saw so many horse explosions and accidents during my time with the serious barrel racers that I figure there has to be a correlation between high stress high impact disciplines and the behaviors that are seen in the horses most popular for those disciplines.
I have assume that there are pleny of appendix QHs that are raised properly with love and care, without soundness issues, that have a lower stress lifestyle that fits their horsenality, who are safe, sweet, and happy.