My experience is they are fairly reliable. Fairly, as in they usually work even after 60-plus years in the large DuMont televisions (er, telesets. Sorry, Balcom) that I restored.
But, they can fail. One of them failed in an RA-119A Royal Sovereign after @20 hours of use. It was the 1.75 MFD 850VAC in the main chassis. I replaced it with a couple of non-polarized oil filled NOS GE 3.5 MFD 440VAC in series.
That left the remaining 1.5 MFD 950 VAC oil filled in the HV chassis. Not wanting to tempt fate, replaced it with a pair of new C-D 3.0 MFD 660 VAC, again in series.
