As far as I understand, you can make an actor reentrant which frees the thread. It's definitely a leaky concern compared to Erlang's process implementation.
My understanding is that isn't really preemption, rather it's cooperative scheduling. As soon as the scheduler hand execution to the actor, everything else in the system has to hope there isn't a bug in the actor that permanently ties up that thread.
Good point. I guess I find it like a hybrid where you don't benefit from the cooperative side and don't achieve true preemption. I think either end would be better than keeping the middle ground.