Earlier quoted context omitted.
I think BeOS tried to and it did not went well with common threading issues poping up on BeOS apps. Not sure if Haiku has any improvements regarding it.
Not sure what you mean when you say "common threading issues"; the entire point of the Be design was to force all threads to use message-based communication instead of shared memory for virtually all uses, and mandatory locking otherwise. It's extremely effective.
It was quite common to see BeOS apps crashing due to not handling the threads communication properly.
Having the whole OS being so multithreading heavy was a novelty for the large majority of developers and quite easy to make mistakes.