As an Elixir enthusiast I would say, if you prefer Erlang to Elixir or is more experienced with it and will be building web stuff Elixir is one more dependency which could br avoided.
I like Elixir. Not all Erlangers do.
Also, this mentions being something of a CMS. Phoenix is not that.
Web frameworks are something where the one objective advantage of Elixir over Erlang really shines: macros.
Phoenix uses macros very effectively to reduce the sort of boilerplate that is common in Web frameworks. Macros aren't the solutions to all problems, but in this case they're great.
I really love Elixir as a language. Phoenix looks nice from a distance, but I found it to be very ugly and messy when you get down to the nitty-gritty. I wrote a personal project in Phoenix but I'm thinking of translating it, because I really want to wash my hands of that framework forever.
Just the general feeling of having to constantly dig through the documentation trying to suss out unintuitive things that would be easy in other frameworks. And poor (imo) naming conventions. And a lot of other little annoyances. No one specific thing as much the feeling of death by a thousands cuts.
That said, I wish it were in Elixir. Has some nice features, but would probably get more adoption if it were in Elixir.