Emergencies always conjours images of engine failures, however of all the pilots I know, engine failures are rare and I don't know anyone who had engine problems in IMC. However, between a small group of IFR mates, we can rack up the following issues in the last 5 years:
Alternator failure at night IMC (luckily a twin)
Alternator failure (single IFR plan but in VFR)
Vac pump failure (single IFR plan but in VFR)
AH failure (just exiting cloud on descent to land)
Pitot heat failure (IMC with ice).
I think its these type of things that are more likely to get you. A battery powered GPS pretty much deals with all of these. This largely echoes the FTDK response, which I think is pretty good.