I have been writing multithreaded code for decades and it is hard. My current database engine can break a single query into tasks to be run in parallel for much faster performance; but finding and fixing bugs is a real challenge.

All it takes is one critical section to not be protected (i.e. locked) to cause a bug. A series of tests can run hundreds of times correctly without detecting the problem. It is only when a context switch happens at a certain microsecond that the error is exposed.

I am a true believer in multithreading as my own code can see tremendous performance gains using it on the latest multi-core CPUs; but tread very carefully when programming in this manner.