The problem is that other languages necessarily place more restrictions on how their data can be used in order to gain all their nice features. Since you can't

The problem is that other languages necessarily place more restrictions on how their data can be used in order to gain all their nice features. Since you can't control the caller you lose all those guarantees. Because of that it will never be easy to interop across higher level languages. C works well here because its low level enough that it expects few guarantees. Just keep the stack aligned and balanced and it will mostly be happy.