Is ballast resistor failure a myth?

spends its life turning anything from 3 volts to 5 volts at 2-3 amps into heat.
it will fail eventually

nothing wrong with mopar system although these days there may be something wrong with the the quality control and the way its manufactured.

on a 12 volt points system that resistance is in the coil...coils fail as well.

its all about limiting current through whatver your chosen " Coil switch" can handle
2-3 amps for points, similar for standard mopar box

way more for HEI and similarly specified 12-14 volt "low resistance " ignition coils with less than say 1.2 ohm primary, i.e 4-7 amps
Hei works with a 0.8 ohm primary

dave