Actually U-brakes are great for off-road riding. It's the positioning on the chainstays that can sometimes lead to mud clogging the brakes in wet conditions. Many mountain bikes of the 80s and early 90s were specced with rear U-brake on the seatstays and these worked very well in all conditions, and flex a lot less than cantilever brakes. It bugs me when people think that the brakes themselves were poor performers when the chainstay positioning is actually the issue.