+1 to rattray's questions. Some more questions: do you think Django is showing its age? Should new web projects be started in new frameworks like Phoenix, or is Django still good to go?
I think Django is showing its age, but not in a bad way, in a mature way. I truly admire the way Django and its core team have matured. They have been role-models for me for years and still are.
Django's momentum is only getting greater. Mozilla has been dumping money into it recently, and important projects like Django REST Framework are getting funded as well. Django is level-headed and forward-facing. I can't emphasize the importance of those two characteristics enough. Look no further than projects like Andrew Godwin's django-channels for evidence. I think starting a new project in Django absolutely sets you up for success.
That being said, Phoenix is terrific so far. It's fast, fun, and the Erlang/OTP concurrency story is really exciting! I'm lucky to be at a place where I can take a few months to really explore it; and I'm loving it! I will choose Phoenix for my future projects, but not because Django is losing any steam.
Of course, no one can tell you whether it's good for your particular business case- but Erlang is battle-tested, and Elixir/Phoenix have made it both easy to reason about and perfect for being productive. If one had the option of choosing a framework right now, I don't know of many reasons to not use Phoenix.
Personally my biggest reservation would be the difficulty of debugging production issues that span two languages - Elixir and Erlang – when I have no familiarity with the underlying language (and when that language is not widely used, and therefore likely to be sparse on StackOverflow).
That's not to say I wouldn't use Phoenix; it's just that that's the biggest drawback I can think of (aside from general ecosystem maturity, which it sounds like is on an impressive clip).
EDIT: Oh, and Elixir doesn't have support for static type analysis (though it does have typespecs[0], and there is Erlang's dialyzer[2]...). Python has mypy[1], and JavaScript has TypeScript/Flow.
[0] http://elixir-lang.org/getting-started/typespecs-and-behavio...
Nor have I heard of anyone else having that issue. It's one of the reasons I enjoy it so much- I get all the performance with none of the complexity.