Hard to get good data on this.
There are good avionics, and there are crap avionics, and all in between. Even within one manufacturer's range you get some good vintage and some real dogs. And hangarage v. outdoor parking makes a difference, although the data on this is not conclusive (IMHO largely because the owner rarely gets feedback on the actual fault).
I've not been kicking around GA for many years, but it seems to me that
- most ADFs are crap, except the Bendix/King KR87 which is very reliable
- most Narco stuff is crap
- the late-1990s-designed Bendix/King (Honeywell) stuff like radios (KX155A/165A), GPS (KLN94), DME (KN83), HSI (KCS55 etc) is pretty reliable, while their autopilot from the same era (KFC225) are hugely unreliable
- the newer GPSs (Garmin 430/530) are pretty reliable
When I used to rent spamcans, must of the stuff in them was duff but the avionics were all old - mostly 20+ years. This is not suprising since I don't think you will get a 20 year life across the board of avionics, by a long way, in a plane sitting outdoors all year.
The Avidyne glass cockpits get very mixed reports, with many crashes of the software especially in plastic planes. The G1000 seems to be a lot better but IMHO it is too early to tell re long term reliability in humidity etc.
Vac pumps do go, not if but when, and sometimes gradually and sometimes spontaneously. I changed one at 700hrs which did not make the green arc below 1000rpm.