Why don't generators transpile well?

We're following the airbnb eslint guide and in it they say recommend not using generators 11.2 Don't use generators for now. Why? They don't transpile well to ES5. I don't seem to be able to find any explanation as to what they mean by not transpiling well (not just in this document, but on Google). We're using babel and there are polyfills to do this. Is there something I'm missing?