I've had a vac. pump fail, losing my AI & DI, 2 x AI only, and 2xASI. The vac pump & 1 x AI were both at night in a part of Oz with stuff all ground lighting. The ASI and 1 x AI were daytime in IMC. It was a bug for one ASI, an automatic/airspeed deactivated pitot cover the other ASI failure and a stuffed bearing (I think) for the AI. The AI wouldn't sit straight ie it was at odds with the flight director command bars, the second AI & the performance instruments.
The vac failure & one of the sole AI failures were at night in a typically minimally equipped C172 ie single of each instrument. The single AI was during a student's Night VFR navex, the vac pump while I was doing the same student's Night VFR flight test. Unlucky ******! He passed, in part due to handling the failure so well.
If it can fail, it will. If you fly using mechanical instruments then sooner or later one will fail. It's no joke about needing decent lim.panel skills including lim. panel unusual attitude recovery. As was rightly said: Have with you something to cover the failed instrument(s), most importantly covering a failed AI.
Whatever you do, don't ever fly something in IMC that doesn't have two independent attitude references. Either two direct sources ie AIs with independant power sources, or a direct + an indirect ie AI + TC/ASI, also independent power sources.