…as is Swift with NSOperation/GCD. Apple's process isolation tech (XPC) is built upon their multithreading tech (GCD). The two are complementary, not antagonistic.
Doing multithreading on a runtime with automatic reference counting gc and expecting performance is nuts. Every reference assignment is a potential write contention.
It is possible to do it, even sensible when you need it for GUIs for example. Just don't expect it to use the hardware in a sensible manner.