A post legged hock is a weak hock. It can be easily injured with heavy work. The angle of the leg increases stress on the tendons and ligaments. It also causes a lack of flexion and can make for a very rough ride. Post legged horses can be very stabby and short strided in the hind end.
Personally, I think this is a very unbalanced horse and is probably wonky to ride. Big front end, little hind end. Her legs look fragile and her back is too long. The muscling in her neck isn't developed correctly either. Her look screams "heavy in the front end" to me. Besides that, pushing off those hind legs like she should be is probably uncomfortable.