I would say it is quite mature. The ecosystem is a bit smaller and younger, so sometimes you'll find an insufficient library somewhere or no library at all. But I've never had a show-stopper in that regard.

I've done plenty of Django and Flask and I'd rather work with Phoenix. I still like Django plenty but only time I'd pick it is if I think the admin might save me copious time or if there is something else tying the project to Python, such as machine learning.