Stiffness Spec
Have a look at the spec sheet.
I do not know what torque you are transmitting, but the 1" part has over 1/4 degree deflection / nm of torsional stiffness.
I could not see a spec for backlash, but it wont take much wear before there is significant backlash.
Even before microstepping, at approx 0.001" / step (1.8 degrees) at half the rated torque you must expect 0.0003". Lightly loaded this may be OK but these couplers will deflect more than a helical coupler.
You will get what you pay for.
After looking at some stepper specs expect over one degree at high torque !
Super X3. 3600rpm. Sheridan 6"x24" Lathe + more. Three ways to fix things: The right way, the other way, and maybe your way, which is possibly a faster wrong way.