Mike, the Doman hub drawing as posted above shows the hub connected to the airframe by a double row spherical roller bearing so all the ”U” joint has to handle the drive torque which is decreased by the ratio of the planetary gear set on top of the hub. So of course an automotive CVJ would suffice for most any small helicopter but not a heavy lift helicopter.
Consider this. A standard U-joint will generate a 2 per rev vibration once the angular difference comes off of 90*. The angular difference causes an increase and 180* later a decrease in rpm on the output side of the joint.
To solve this problem. add a short shaft and a second U-joint. Now you essentially have a doubled 2 per rev vibration, but the angular difference are equal to each other 180* apart. So the end result is a cancellation of the 2 per rev vibration.
Or, think about it this way:
your rotor mounted on a single U-joint would require a lead-lag hinge to compensate for increase and decrease of rpm difference between the rotor hub and driving shaft. If you add a short shaft and a second U-joint (CV joint) then the rpm change would be cancelled out within the joint and the driving shaft and rotor hub would not have a 2 per rev vibration and no need for lead-lag hinge
This is the same principle applied to a front wheel drive automobile. A CV joint takes care of the problem. If you ever drove a 4 wheel drive vehicle on asphalt and turn the steering wheel. You can feel a side to side pull within the steering wheel. That is the 2 per rev vibration back into the steering controls because the tire (rotor) cannot slip. If you build the drive joint as a CV joint the 2 per rev vibration is cancelled out. Hence driving a normal front wheel driven vehicle in a turn - smooth.
Hopefully this will allow you to visualize this problem.