For language coverage, Function currently doesn't support classes (on the roadmap) or lambda expressions (much harder). But these are the main limitations.
For library coverage, this is where things get incredibly hairy. If a library isn't pure Python (i.e. most of the important libs), we can't compile them. We instead have to reimplement the library's functionality. For now, we plan on using LLMs to automate this as much as possible.