One other nice thing with these motors is they give you the perfect register to locate from and keep the shaft concentric. I concour that mounting each motor to the frame and keeping everything aligned is basically a disaster once you start to put a load on them.Once aligned you can then drill a couple of holes in the flanges and insert dowels, this stops it from shifting and means it’s perfectly aligned if you ever need to dismantle it.
If you don’t have it rigidly mounted and well aligned you will end up with significant coupling wear regardless of which style you use. It will also induce vibration and put extra load on the motor bearings leading to increased heat and wear. The tolerance for alignment becomes tighter as speed increases, not sure what your planned speed range was but even at 2000rpm I’d want to see no more than 0.05mm misalignment.