The coupler should be as short as possible otherwise the windup effects mimic a long spring. With modern servo systems, the control system can induce very slight vibrations. The pulse width modulation often used to control the motor velocity might be felt. The control system algorithm may have periodicies and the feedback loop may induce vibration under certain load conditions. With stepper systems and their typically highly geared drivetrain, a somewhat flexible coupler can help smooth out any jerkiness in the stepper motor as it moves from step to step.
Stepper motors snap from step to step when operating in fullstep or halfstep mode.
This causes a jittery motion in the eyepiece that can be quite objectionable. A good test for this is to turn the drive off and on to see if there is any difference in the image quality. Another way to test is to put fingers lightly on the eyepiece. Jittery and higher speed oscillations can often be felt, if not downright heard by the ear. Metal tubed telescopes such as refractors don't have the natural frequency absorbing characteristics of wood and aluminum Dobsonian mounts.