Is it the wrist angle, the shortening of effective stem length, the lack of reach, a bit of both or something else that makes the Jones bars less good for fast steep gnar?
The narrower width I’d say, though a 710 H bar feels effectively wider to me than a 710 flat bar. Grip reach shouldn’t change with the H, you should account for it with stem length, plus you get more fore-aft grip range on a bar with that much sweep. On the downhills you’re at the rear, widest part anyway. If it’s fast, rough and you’re trying to keep a susser on line a wider flat bar and more elbows-out position just feels more braced and stable.