We have had major issues with aftermarket starters. We've also had problems with mufflers where the pipe that are welded onto them are not the correct diameter and instead of the technician alerting us about it so we can return it, they mess with it for an hour and a half to get it to fit. Most of all we have good luck with aftermarket but we are selective about what and who we buy from.
There are some companies that will cross reference a temperature gauge for instance. Their feeling is that it is a temperature gauge and if it doesn't fit in the hole it doesn't matter, it's a gauge, that's what matters.