I don't know how good a teacher he is, since I've never been to any of his classes, of course, but his planning of Charlotte's career with all that travelling around, is obviously disastrous.
There is something really weird about his relationship with Charlotte and that's the simple fact that he trained her when she was a teenager.
Think about it: Charlotte decides to go riding at the Rozier's stables. That's normal. She's just a child, she's starting to jump, doing small competition, 1 m, 1'05, 1'10. I'm sure the Roziers had many pupils of that level. Usuallly, in a big barn like that, the big names, like Thierry Rozier teach only advanced pupils. The children are left in the care of younger teachers, good riders and good teachers, ok, but no the top ones. An international rider doesn't teach beginners! What was he doing training Charlotte by then?
And yet, whe had Thierry Rozier always stuck to Charlotte, and Valentine, and Caroline. And since Charlotte and Valentine took part in small events around Paris, he stayed with them, he was always at Fontainebleau or Bois le Roi with them (didn't he have any international shows to attend?)
Then when Charlotte comes back to riding and decides to do the international big thing, Thierry is again with her. Wasn't him in charge of the children and the competitions near Fontainebleau? Where are his other pupils? No. He makes Charlotte buy a lot of expensive horses (I'm sure he was the one who conselled her) and he advises her to ride the GCT (he said so in an interview) and there he is again, always with Charlotte and Caroline, with the Guccci people, in the magazines, giving interviews... and again, where are his other pupils? He seems to have only Charlotte!
I really don't know him and I don't know how good a teacher he is. But he strikes me as someone who wants to be close to Charlotte and parade her around because of all the free publicity he gets from her, and also the kind of teacher who will never tell his wealthy pupils the things they don't want to hear just to keep them with him. The kind of person who will never tell Charlotte: you are not ready for the GCT and don't even dream about the Olympics.