I really shouldn't post messages/comments when I'm tired and not thinking correctly ...
Ballast resistors aren't actually unreliable. They are reliable and rarely fail. On the rare occasions when they do fail, it's very frustrating. It's also annoying, considering they're inexpensive and easy to replace.
The factory original in my '77 B200 van lasted between 2-3 years and thousands of miles. (I bought the van new, with 2.8 miles on the odometer.) I don't know long the replacement lasted because it was working just fine when I sold the van 6+ years later. (By then, I'd rolled over the odometer twice and approaching a third time.)
I am not aware of one failing while the engine is running and the vehicle is moving. What I know about electrical circuits and ballast resistors leads me to believe they only fail when you're trying to start the engine.
Most electrical circuits experience a very brief surge of electrical current when they're initially energized. Ballast resistors are a component in a vehicle's electrical ignition circuits. The 'shock' of this initial surge, when the ballast resistor is cold, is the most likely cause of them frying. ('Cold' is relative, mine failed when it was 90 degrees F outside.)
So, the ballast resistor is not something you really need to worry about. Instead, I make my recommendation for carrying a spare in order to help you avoid a potentially frustrating and annoying experience.
Note: My recommendation has the implicit assumption you also carry tools suitable for dismounting the old resistor and mounting the new one.
