Hello;  I asked this same  question when I was apprenticing to be a
mechanic in 1980, the answer I got was that the traditional u-joint will
oscillate if forced to  run beyond a maximum of 16 degrees of deflection,
this oscillation is because the yoke of the joint tries to accelerate and
decelerate through its rotation.  The explanation came with training aids
and a demonstration of the oscillation.  The constant velocity referred to
is meant to imply that this demonstrated oscillation does not exist.  This
was also demonstrated to the entire class.  It was a lesson that I'll bet
no one who was there will ever forget.  Another benefit of the CV joint is
that it will work at a higher deflection  than 16 degrees, I don't remember
the exact number.  Another little known fact about U-joints is that the
deflection at one end should/must equal the deflection at the other end,
for the shaft to work smoothly and not wear excessively.
                              Larry Carter