There should be no reason to use python2 over python3. Your argument:

> I've been avoiding Python 3 in production out of concern for poor library support

Is the exact problem the python community has at the moment. If everyone takes this stance then the projects will never be move forward.

Why not make the point of using python3 and porting or fixing code where it is broken. Tools like py2to3 make moving from python2 to python3 so much easier.

Your question:

> but what scenarios is Python 3 inappropriate for?

Python3 is inappropriate for the situations where python as a language is an inappropriate solution.