[Dave Bilyk]

Ron,
looking at Big Foots animations, I can see three factors that might limit the life of a CV joint.
First, the balls operate at a larger radius than the UJ bearings, which means more movement for a given articulation.
Second, the balls try to ride out of the grooves, and require high stiffness in the outer part to control this tendency.
Third, the balls have to transmit force while some sliding takes place on the loaded faces, which means more friction and sliding with the larger movements mentioned above.

As Jac Mac has said there is a significant difference between a UJ / CV combination, and a UJ / Rubber Donut combination. The flexibility in the Donut allows harmonic movement of the shaft without generating significant harmonic torques, while the CV is stiff, generating harmonic torques which are fed into the wheel and back into the transmission.

regards
Dave