I've personally never had an MSD unit fail*, but I've been around several that did fail. A couple 6's, one a 6OR, and at least one 7 that we had on the dyno for those engines that needed such a box. This is experience going back into the early 90's, so not likely counterfeit units.
If you do use an MSD then do yourself a favor and cut off their proprietary inductive pick-up connectors and replace them with a weatherpack or a metri-pack or a deutsch, anything that is sealed. If you won't do that then at least wrap the whole connector from one wire to the other with 3M's 130C tape. If you don't do one of these two options then eventually the corrosion on those contacts will get to the point where the box won't work because no signal can reach it.
*I own two 6's, they're on a shelf somewhere as I don't use them and haven't for decades. I should probably throw them out the next time that I stumble across them. I'm not impressed with them, the Performance Distributors HEI module runs just a well and is a whole lot smaller, less expensive, and less picky about the surroundings. Wire an MSD to an inductive pick-up with reversed polarity and it usually runs very badly. Sometimes it won't run at all. Do that to an HEI module and it doesn't care. Use cheap suppression type spark plug cables around an MSD unit and it does odd things, sometimes randomly, sometimes consistently. Do that with a D-S II or HEI module and they don't care.
