Does a coil wear out, or is it either good or bad?

 

Theoretically, no. Practically, yes. The coil windings themselves don't "wear out", per se, but the insulation around the windings can degrade, affecting the coil performance. Silicone oil filled coils seem to last the longest. Epoxy filled coils can last a long time, but tend to heat more quickly, and the heat is what accelerates the breakdown of insulation.

Having said that, it would seem a bit ironic that the oldest coil I have is an epoxy filled coil in a Fairbanks FMJ magneto, mounted to a 1938 Allis Chalmers B tractor. It's original. I'm still waiting for that coil to fail, and if it's within my lifetime I'll let you know.

However, larger coils (transformers) that are oil filled can last much longer with less insulation breakdown. Many large transformers have sample valves to tap off an oil sample for analysis, and properly cooled and maintained wet transformers will far outlast dry types. The same philosophies apply to a smaller transformer (coil).