There are two primary costs to preemptive, kernel-driven context switch: first is the userspace to kernelspace transition cost which, while has been going down for a while, it is still non-trivial and actual got worse with the various spectre mitigations. Second cost is that preemptive context switches must by by necessity conservative and need to save and restore a lot of state that might not be necessary on a cooperative switch.

Stackless (i.e. async/await as opposed to green threads or stackfull coroutines) context switches have the additional advantage that they reuse most of the stack and only suspend a single stack frame. This means that the rest of the call stack (shared between contextes) can stay hot in cache.

Whether any of these costs matter depend a lot on the application, the amount of context switches and the amount of work done between switches.