Look at the two shaft assemblies below. Identical, right? Not exactly.
Another name for a Lego universal joint is a Cardan joint. One of the drawbacks of a Cardan joint is that even when the input axle rotates at a constant velocity, the output shaft rotates at a variable speed. This can cause vibration in shafts that turn fast and inaccuracy in precision machinery. It’s not typically noticed with Lego because of several reasons. There is not a good way to very precisely measure rotational velocity and angle, there is torsion (twist) in the shafts and the gearing has a lot of backlash or slop, so we don’t see it or even care.
One way to partially overcome this error is to align the hinges on the universal joint as shown on Shaft A (90 degrees apart) and align the input and output shafts so that they are parallel. This is called a Double Cardan Shaft. This arrangement will result in an almost constant velocity in the output shaft.