I agree with readme needs to specifically mention unimplemented features. But the responsibility for using an unpopular library lies fully on the user, not author. Of course if the library is popular, author needs to maintain it, or delegate auhority to other maintainers.

As user, it's clear that you need a feature which you found on an unpopular library. Now rather than be skeptical and research the library, or wrap it under yours so you can replace it later, or develop your own, you decide to blindly use it and blame the author.