Immutability and thread-safety - if you want to get a value from one concurrent process to another, you have to copy it and send it to the other process. You can circumvent this by using a smart-pointer / reference to mutable data, which is manipulated by native code. One example would be a ResourceArc[0] in Rustler.

[0] https://docs.rs/rustler/0.22.0-rc.0/rustler/resource/struct....