A yaw damper?
No 4- or 6-seater piston plane I know of needs a yaw damper. The longer-hull planes (e.g. a TBM, a PA46) have them and they need them for passenger comfort.
I looked at installing one in my TB20 because it should reduce the roll excursions in light turbulence (for my girlfriend, mostly) because of the yaw/roll coupling which all planes have, but it is a rare option on my autopilot and would cost well above £10k.
On the other stuff, people with engineering/technical backgrounds seem to transition to the more complex types more easily.