I have now ten or so in operation, and only one, a very old one with the pre-laserwelded plug cap, has a bad cap (the soldering broke). It still works for testbed running, but better not touch the cap while running (have to admit, it is not near as "shocking" as 10+kV would lead to expect, but still not pleasant...
I had two suspect failed ones, that turned out to be borderline worn out ignition batteries that would not maintain voltage under load. Among them the 5-lead radial ignition. A new battery with one cell more (5S NiMH instead of 4) completely cured the issue.