Jaw couplings are crap. They are designed for shafts that are out of line in one axis, not two axis as often happens in home built machines.
Any out of alignment takes up power to correct, run one at say 10 degrees out and see how hard it is to tun and what heat is generated.
now transpose this into two axis and that creates problems. Jaw couplings or Lovejoy couplings to give them their correct name are not designed for zero backlash, they might be well made but they are not guaranteed to be zero backlash.

The only type of drive that can handle miss-alignment in two axis is the Oldham coupling