I think the primary benefit of hydraulic brakes, is less effort to brake. With hydraulics, the braking power is_not really dependent on the effort that can be applied by the rider; fairly mild pressure on the brake levers, gives good / max braking. Depending on riding style / conditions, that can be a big plus. Not necessarily specific to downhill riding.
Personally, having used (mech) disc brakes for few years (on my Trek 4300D), I felt v-brakes were better overall, for my riding. I felt the brake pad alignment (which probably is equal effort, for mechanical and hydraulic) in disc brakes is a bit of a hassle. Though it might also be partly dependent on the specific brake model.
-{db}.