I agree with both of you. I don't think threads are THAT hard to work with. It definitely takes some experience to do it well and quite a bit more documentation to maintain the expected invariants. When libraries can get into a tangle, it's usually code that's in house and better ripped out. Easier said than done I know.

Open libraries tend to either just be single threaded abd should be used as such or explicitly thread-safe.

Disclaimer: Used threads in Java not much in C. Love me some Jsr-133 volatiles. Still confused with the Java 9 memory model updates.