The real trouble of using Emacs C core from another language is that you have to use all the internal datastructures used by C core, and this exactly equivalent to all elisp datastructures.

This implies you have either to convert back and forward everything or you just can't use the native datastructures of the new programming language (making the whole operation often quite pointless).

I must confess that most of the comments in this thread seems to start from the assumption that people working on this in the last two+ decades are probably dumb, and this is sad.