My Arab x Welsh pony is a great low level jumper and dressage pony. I've jumped courses on her up to 70cm, single jumps up to 80cm. She could go miles higher, (but I can't!) her form is good and she enjoys jumping. Her dressage is nice and soft, I think I could take her up a few levels if I was a better rider. We haven't had any problems in the past wth collection or hind quarter power. Our biggest dressage problems are walking out more and leg yields, our biggest jumping problem is the random flying lead changes she likes to chuck in when shes excited. (No, I can't figure out how to get them on cue)
However, shes is 3/4 arab and all of the Arab bits are from colonial lines (endurance/racing, not showing) so she naturally has a bit of strength there. And her mother (1/2 Arab, 1/2 Welsh) was an A level show hunter so thats probably where the jumping came from.