Top breeding in that horse. From my perspective, the disadvantage isn’t having "xx’ in the pedigree or even being full-blood that edges your competitiveness in dressage. It’s whether the horse you’re riding raced. Those horses have long term physical baggage from a short-term career – AP Prime, Unmarked Bills, are all at the ULs in spite of their racing career… and that is saying something.
I think people forget that when they look at a TB at the top level and think the horse in front of them would be the same horse if unraced. In my experience, that’s not the case.