They're heavy (and they either need a booster plate or beefed up stays to work properly meaning even more weight) but their biggest sin is to clog like you wouldn't believe.
The extra braking power is a bit pointless on the rear, especially for how we ride today. In the past it made more sense as people would hang way off the back on steep descents so that the rear wheel would be forward of the CoG and thus a powerful rear brake had a point. U-brakes were ideally suited for this as they don't stick out like cantis and so doesn't jab you in the thighs while hanging off the back.
The extra braking power is a bit pointless on the rear, especially for how we ride today. In the past it made more sense as people would hang way off the back on steep descents so that the rear wheel would be forward of the CoG and thus a powerful rear brake had a point. U-brakes were ideally suited for this as they don't stick out like cantis and so doesn't jab you in the thighs while hanging off the back.