Mainly export settings and so on.
I’ve only tested with TTF-based sprite fonts at the moment. If you wanted to support a bitmap font, you’d need to copy the implementation of FontAssetCompiler and use the MonoGame APIs to build the underlying XNB.
If you have an example in the MonoGame code base where it compiles a bitmap font, you’re basically looking at copying that logic into lines 65-71 of your FontAssetCompiler variant.
Also, I’m happy to upstream this work, so if you get bitmap font importing working, send a PR and I’ll merge it in.