Vibration is the biggest issue, and the knuckles accelerate at different (un-timed) rates. The more the angle, the worse the vibration. Vibration leads to bearing failure on the cross.
In your PTO application, steeper angles can cause more issues. TYPICALLY, big PTO shafts are triangular cross sections with a flat on one corner, so the joints cannot be misaligned when reassembled....UNLESS, they were repaired incorrectly at some time (welded out of time). Also, if the shaft was fully collapsed (no available telescoping) the drive line would operate very roughly.
Ideally the center of the driveshaft would also be over the center of the pivot point (then in a straight line) to split the drive angles equally on both joints during a turn. This is more important with towed implements like batwings, mower conditioners, balers, etc.