It can be useful to directly unit test edge cases in critical concurrent code, because they are otherwise difficult to test deterministically. But like you said, I've also found them difficult to write and maintain (re-reading some of them months later is usually hard). The tests usually end up with 2-3x channels than the prod code, because I'm forced to inject channels into various places to control the synchronization. Sometimes though, in a critical code path, it's worth it.