Q re attitude-based autopilot (AP-3C) and GPSS

I read a review of the Chelton AP-3C recently, and it has me wondering some
things.

It's attitude-based. The article I read seems to work on the presumption
that this is better than rate-based, but it does later say that one
shouldn't be dependent upon a vacuum-based AI for one's autopilot.

So what do people with attitude-based APs typically do? Electric AI? Which
electric AIs don't tumble?

I'm also wondering about the ability of the AP-3C's ability to use GPS
steering in both horizontal (GPSS) and vertical (GPSV) planes. If on a GPS
flight plan, what would occur if the AI failed? If the AP is working to
keep to the flight plan (in both planes), wouldn't the airplane stay
straight and level even if the AP thinks that the plane is in a descending
turn (or whatever it thinks from the failed AI)?

- Andrew