> If it's a type you're having to invent as part of writing the code, writing it out everywhere it gets used can be a lot more work.

Not really; you write it out fully once, when creating a type alias/definition, and then just use the name later.

> Meaning, not well documented compared to APIs in other languages?

Yeah, I’ve found Python to have a pretty good documentation culture (docstrings especially facilitate this), often better than some statically typed languages with ecosystems where signatures are regularly mistaken for adequate documentation.