Correcting your correction. A u-joint can run at constant velocity if it's installed with zero offset between input and output shaft. If this wasn't the case, the single u-joint in your Mercedes would be driving you crazy with vibration. In a typical non-Mercedes installation, installing a pair of u-joints out of phase allows the input and output shaft to be offset: since the speed variation of one joint is exactly the opposite of the other, the wobbles cancel out. A CV joint can be considered as a pair of u-joints in one assembly, either literally as in a double cardan joint or nested as in a Mercedes rear axle CV joint.

If a u-joint runs at zero offset, lubricant is distributed just fine. But what happens is that the rollers always track over the same path in the races. As a result, the races develop a typical wear pattern called false brinnelling. This is not a problem unless the joint later needs to compensate for a misalignment as would happen with worn mounts. In this case, the joint could become noisy, as the rollers track over the groove worn in the races.