Semaphores are more general for sure. The common semaphore is just a counter, when you lock it you decrement the counter by one atomically. Therefore a mutex is just a semaphore with a limit of 1. I wouldn't say they're much trickier to get right, maybe just less frequently applicable in e.g. general web io tasks.

I suspect the reason that semaphores aren't as common in Rust specifically is that once you go above one reference to something, you stop being able to mutate it safely. Mutexes work because "one thing writing to this means nothing else can read it" doesn't go against the borrow-checking grain, but as soon as you want to allow two references to something, you can't write to it safely, so it only works when you want to delay processing of non-shared data (which as you mention is not particularly common) or read-only processing (at which point you probably just want an RwLock to get shared read-only concurrent access).