Ive used M+H on maybe 5 or 6 cars so far. Ive had 2 that had issues.

One was the feed wire for the ballast resistor was crimped over the insulation instead of bare wire 1968 B body ( thus no voltage to the coil )

2nd one was a mis-wired bulkhead connector in an E body for the wiper operation ( had 2 wires crossed up )

That said I will not hesitate to use them again and again.