Its downsides depend on the language. It works better in languages with GC than languages without, and it works better in languages that rarely rely on FFI than

Its downsides depend on the language. It works better in languages with GC than languages without, and it works better in languages that rarely rely on FFI than in languages that rely on it a lot. One of its major "downsides" is that it's just harder to implement than async/await, and requires control over the backend, something few languages have (even Rust is on top of LLVM). For example, Koltin just couldn't implement useful usermode threads because it has no control over the JDK.