In an individual race, there is very few instances where the rider makes the difference. In most cases, the best horse will win and any of the top 50 jockeys could do a job. I do think that there is a cumulative effect that ends up with the best jockeys making the best decisions and ending up on the best horses. (And over time, those not good enough losing those rides.)
There are other factors other than race-riding that ends up with the best jockeys on the best horses. Bryan Cooper is one of those jockeys who would be capable of winning on most horses but lost two high profile retainers, Sam Twiston-Davies likewise. I would rate them marginally worse than the best and went through a long period where a series of marginal decisions ended up with them losing good roles.
Geraghty reports in his book that the year before retirement, JP called him in and reported that trainers were complaining about the rides he was giving and whether he was thinking about retiring. Even with the very best, its noticed.
I think Bryony is as good as most out there and not a negative on any horse. I don't think she stands out as one of the top five jockeys (for instance), but not sure there are many who claim her to be.