Two of my unicycles have V-brakes (via adapters on Magura mounts); one is a 26" muni and the other is a 36". The V-brakes are cheap, easy enough to adjust, take a whack well, and are less worrisome being mechanical.
My 29" has a hydraulic disc. The braking action is smoother and it has more power. It’s never given me any trouble but if there was a mechanical caliper available that was as narrow as the hydraulics I’d switch it just for piece of mind.