IMHO the design of them makes them weak, for one you can see whats left when a diff cup breaks and you can see exactly how much metal was actually holding it together (NOT MUCH!!) the slot they machine into it weakens it too much as there is not enough metal down the bottom end of the slot.
And with the CVD's its the same story when they break right up by the universal, again take a look at exactly how much metal was actually holding it together, with the CVD the slot they machine into it on the ball part actually weakens it substantially as the slot pertrudes all the way into the main shaft.
As for them twisting and breaking (at the dogbone end) this is caused by them being too weak because of: a. the materials used or b. the overall thickness of the shaft (although the thickness can be overcome if the material used is stronger).

As well as this you have the company admitting they are weak! What more do you need!!!