You are right, it is very difficult to ride a "Schoolmaster" absolutely.
I would consider Nelson one - why, you may ask? Because my opinion, a School Master is a horse that has gone to a level where you have not, and done successful at his/her sport with his/her rider at that level.
For example, Nelson went A Circuit Hunter/Jumper with his first owner and became a very well known horse that wins in the LMHJA. And then he was sold, and went to a dressage barn where his 2nd owner competed him to 3rd level dressage, and then sold him and his 3rd owner took him Prelim Eventing.
I, have not gone A Circuit Hunter/Jumper. I have not gone to 3rd level Dressage, nor have I been anywhere near Prelim in eons in the sport of Eventing.
When I met Nelson, he was stunning! Just stunning! His previos owner, took lessons from his dressage owner and watching their lessons made me drool. Nelson would be soft, supple, light, rounded, engaged, moving beautifully - then, when I got on him.....it was crap.
Back dropped, head high = CRAP. I couldn't get him to soften. I couldn't get him to track up, lighten up, round up - nothing. He growled at me alot too - LOL.
Then his previous owner would get on him - and BAM - BEAUTIFUL.
I mumbled profanities.
But, over time - I learnt his buttons. He learnt mine. I learnt how to ride him the way he needed to be rode.
We've been together 3 years now, it'll be 4 this April - and I still cannot get movements out of him, that his previous owners got out of him.
It is very hard to ride a "School Master" if you don't know how to. If you don't know how to ask for the movements, how to ask for those rides that the horse is trained to compete at - it is difficult.
Makes you soon realize, how uneducated you are. That isn't a bad thing - that just means, you have alot more to learn.