In my case, the mirror shift while focusing moves planets completely off the sensor when imaging with a 3x barlow! Without the flip mirror, its a real pain to get the image back on the sensor. I have a Hutech helical focuser to combat this - and you can motorise them as well, but mine is still manual. The other problem with SCT focusers is the "play" in them - you don't get any focusing movement of the mirror when you reverse the rotation of the knob for a small fraction of a turn. So fine focusing is a problem even with motofocus.

With a APS-C size sensor, the image will not appear to move very much, but the tiny sensors on panetary imaging cameras are a whole different story. These sensors are tiny (my Sony ICX-618 sensor is 2.7 x 3.6mm) - an APS-C sensor is a massive 14.8 x 22.2mm by comparison.