People use hydraulic rim brakes on unicycles because V-brakes are hard to fit over 3" tires. Rim brakes on a 36" can rub due to frame and (more so) wheel flex when climbing. V-brakes allow you to adjust the pads further away than Maguras but a disc eliminates that concern completely and give better modulation due to the smaller rotor (rim brakes use the rim as a giant brake rotor - more power but less modulation).
Edit: a bit more on V-brakes vs Maguras: Why Magura rim brakes?