You need to be able to link your pthread condition variable implementation to your pthread mutex implementation such that a pthread_cond_wait() does an atomic wait-on-condition-variable-and-unlock-mutex, noting that another thread can signal the condition variable without holding any mutex.

In addition, pthread mutexes can be statically initialised which requires some kind of initialise-on-first-use for those if you implement them using Critical Section objects.

I believe WaitOnAddress(), rather than being a "competitor" for futex was actually added (or at least, the underlying infrastructure was) in order to be able to implement futex() in the WSL.