Well in a non lifted vehicle the slip yoke is not a bad design and fairly strong (the longer the shaft, usually the weaker it is, and the slip yoke is longer). When you lift a vehicle the angle of the shaft puts undo pressure on the slip yoke. The movement of the driveshaft is vertical but the slip has to occure in a horizontal plane (not good, more angle the worse it is). A fixed yoke with the slip in the shaft compensates by allow the slip in a dual plane (vertical and horizontal). This creates less stress on the joint. Also when operated at extreme angles and droop, the slip splines tend to wear much quicker on the slip yoke and vibrations are a bigger problem because of the above metioned problems. Again, it's not a bad design for a stock vehicle with stock ride-heights and drive-angles, but for lifted and driveline taxed vehicles a fixed yoke is better.