I've seen a few spectacularly unairworthy autopilots (a part of my latent mistrust).
I've owned one of those! My plane came with a Century 31 (I think - it was a while ago). If it had worked, it was a great a/p - it did everything an STec 55 does, coupled approaches, climb/descent control...
It was old and analog, and I think there was quite a bit of corrosion in the internal connectors. I spent thousands trying to get it to work properly - amazingly, my local avionics shop knew it reasonably well. We replaced boards, cleaned things... the last straw was when I was flying a coupled approach (flight testing in VMC) and it suddenly decided to go for a 1000+ ft/min dive at 400 feet.
That was the end. I replaced it with an STEC 30, which is perfect for what I want. I'm happy to hand fly climbs and descents, and on local pottering around, but for a long XC trip it's a lot easier to have the a/p. An STEC 55 would have been a better fit for my plane, but I wasn't ready to splash out another $10-15K.