Most swing style gauges ( not mechanical ) are basically volt meters.

A voltage makes them read higher, but gravity or a spring makes them return to default.

A stepper motor controls all of the needles movements using motors. Whether that be up, down, whatever.
So I guess they have the potential to be more accurate. Or more prone to problems.

As for the Stack stepper motors. Ive only ever seen two stack rev counters which were stepper motors. If all of their gauges work like that, I'll never own one.
Very jerky, slow and not nice at all. Given the price of them, quite terrible really.

When I seen the first I just assumed it was faulty....but then I seen a Stack dash which was exactly the same.