Is ballast resistor failure a myth?

Not a myth- ANY part can and does fail. The frequency of failure has undoubtably been greatly exaggerated over the years, though. As said, many times it is the connection, ground or wiring that plays a role in any issue that surfaces.
I always have a toolbox in the trunk, and a spare ballast doesn't take up any appreciable room in it- if needed, it's there. Same goes for an ignition module and VR. Heck, it takes up less room than a spare tire, and when's the last time you actually used one of those?