Hi! Thanks for the write and for sharing 
I posted a question a few days back that seems on topic, but which, afaict, is not answered in your post (which, iiuc, is target at reason/rescript users more than OCaml users):
As a Jsoo user, my naive assumption was that anything that didn’t involve system interactions would compile, but this seems to be the case. I couldn’t find any readily available documentation explaining these limitations tho, nor what is required to make dependencies available for code that comiles via melange. Any pointers here would be helpful and gratefully received