I agree, I don't see a measurable advantage using universals. I'll play devil's advocate and highlight some of the difference that can be debated. IMO differences between CVDs and universals are below:

CVDs are:
1. rebuildable
2. not as compact so maybe more rotating mass
3. sometimes uses a larger inner bearing on the hub to capture the cvd pins. This larger bearing may have more rolling resistance than a smaller one.
4. the location of the drive shaft pivot point can be different, so dogbone plunge and the operating angle of the joint can be different.
5. Both pivot on 2 points and 4 friction points, but the surface area on the pivot points is more 'even' with the u-joint. The CVD pivots on a small pin at 2 points and a larger diameter 'drive shaft coupling' on the other 2 points.

The U-joints vs CVDs debate on which is freer or has less resistance, or binds less at at extreme angles will get you many opinions which can be misleading because I believe they have nothing to do with how the actual joint functions. IMO some of the performance differences may have something to do with the 5 points above.

If anyone answers my next question, I'd be inclined to possibly change my opinion My question is WHY? why are they freer? Why do they bind less? IMO, the actual joint is functionally the same on a CVD and universal. In reality the parts we call CVDs in the RC world are just rebuildable u-joints. They don't spin at a constant velocity like a real CV joint. An interesting questions would be why are they called CVDs?

A quick google search has many links explaining how a CV joint assembly functions and how they are different to universals.

http://en.wikipedia.org/wiki/Constant-velocity_joint
http://www.imperialclub.com/Repair/UJoints/terms.htm