I've never read Parelli and have only seen a few clips of video. I've got one book by CA. I read it 4 years ago and haven't re-read it. My wife bought a mare that had never been ridden and we hired a trainer to break her. I later rode the little mare out, and then the same trainer worked with the mare & my daughter for riding lessons.
Our Appy gelding was spurred viciously at a ranch he was loaned to just before we got him.
He was very erratic to ride, and we eventually let him stand around for 8 months. Then we again hired the same trainer, and he spent 5 weeks at her place. He came back a bit timid, but ready to ride. Again, I rode him out first until my daughter was ready to take lessons on him. He is now an excellent horse - willing to move out if asked, willing to loaf if not asked, and always determined to stay between his rider and the ground. He is easily our best horse.
Mia & I have been at it for 4 years. I couldn't count the spooks and bolts we did. Eventually, she was so spooky I stopped riding her, for 8 months as it turned out, until I felt my riding had improved. I then hired the same trainer, who was very worried about Mia for the first 4 sessions. On the 5th, she concluded Mia had never been broken at all, which gave a way ahead. She came over 4 times/week for 3 months, and twice/week for 3 more.
Sorry for the history, but it is needed to understand why I'm going to say what follows:
One of the things that really bothers me about the NH video trainers I've seen is that it makes people think they can train a horse when they don't have enough experience at reading a horse. I now have watched a trainer working a horse for countless sessions, and have had her supervise me doing round penning and have ridden out several horses. If someone asked me to do even elementary horse training, I'd tell them to go get someone who knows what they are doing.
As I watched someone who has lived her entire life with horses work on desensitizing Trooper and later Mia, I realized that doing it successfully depends very heavily on reading the horse's mind and emotions and pushing enough but not too far, and knowing when it is time to back off. And I mean time as in 1-2 seconds of accuracy.
After 4.5 years being around horses every day, I wouldn't feel confident doing that with a horse. Get the timing off by just a little, or misread the horse's emotions, and you can take 10 steps back in 2 minutes.
From a 'horsenalities' perspective, if you really need a chart to see it, then you probably aren't ready to train a horse. Not without supervision. At least, not a 'problem horse'. Once someone has enough experience to train a non-problem horse, then maybe the stuff Parelli (or Lyons, or Anderson) teaches may help a new trainer make progress. I can easily see any of these programs as a 'teach the teacher' type system. The trainer who has done so much for my horses (and daughter) was taught under Lyons program, although she has continued to study with others to broaden her outlook. But she went to Lyons having grown up with horses and competed in barrel racing. She had 20+ years around horses before trying to learn a systematic approach to training them.
I object to the marketing which tells people you can learn to train a horse even if you are brand new to horses. You can certainly ride them out, and give them experience at what they have been taught, but there is a big difference between that and actually training a horse - particularly one with bad habits or bad training.
Give me another year or two, and I might be at the point where I could read a horse well enough to START using Lyons or Parelli or Anderson to learn about training a horse. My guess about why people get 'stuck' using Parelli or other video trainers is that they just can't read a horse well enough to have any real chance of success in training one. And a personality chart won't solve that!