Discs are generally better than rim brakes in wet conditions. For absolute braking power, I have found little difference between cable actuated discs and well set up V brakes. Good hydraulic discs, in my experience, have more reliable braking power and better modulation than any rim brakes. The difference in dry conditions is not that great.

However, U brakes are, in my opinion, the bottom of the barrel. I hate them. It is possible to get good braking out of them but they are the biggest PITA to set up, and they limit clearance for tires, fenders and/or mud more than V or canti brakes.

Although my hatred for U brakes might be residual anger from the late '80s when almost all MTBs came with U brakes mounted under the bottom bracket for one or two years. The worst possible combination of an impossible location to access for setup and maintenance, constant buildup of mud and debris (sometimes to the point of jamming the rear wheel), and all this trouble resulting only in mediocre braking.
V brakes were introduced only 5 years after the Great U Brake Experiment was shown to be a complete failure.