It's a little hard to judge her SI joint position since the photo is a bit from the back, instead of exactly perpendicular to her barrel.
But, while her hip looks a little small, her back isn't really long. Her wither is 'deep', meaning it's well set back, and gives her a nice, powerful shoulder. She seems to have a pretty deep heart girth, too. Hard to tell if her ribs are 'well -sprung'. Those are all indicators of stamina of wind.
I like her short front canon bones. Her front hooves are run forward, but will be easily corrected with a few trim cycles. Overall, her feet, like most Mustangs, look amazing. She has nice hocks, yes , a tiny bit over straight.
Her rounded buttocks are so typey for a Spanish type horse. And, her Roman nose, too. I think she throws back to her Iberian ancestors. . . . or, perhaps I'm using too much imagination. Is she from Oregon?
Overall, I like her build immensely.