In terms of language, Phoenix is a complete replacement for Rails for me. I feel more comfortable growing a functional codebase, and the BEAM means I don't have to worry about scaling as soon as I would with Rails. I think it's a good fit for when you want to do more with a small, experienced team.
I would reach for Rails when I'm concerned about finding developers (Elixir devs are fewer and more expensive, generally), or if there are Ruby libraries I want that aren't available in Elixir.
I fear it would be difficult to hire Elixir people as well, but now I realize it's actually also hard to hire decent Ruby/Rails people. Honestly I think we should just accept anyone who can demonstrate thinking and programming skills of any language and then plan for 2-3 months of ramp-up time to get them into our language of choice.
I'd be happy (if I were job-hunting) to be able to try out a different language/platform every so often. Now and again I'll play with, say, Ruby or Elixir or whatever. But I know that I'll never get a job working with these languages because I don't have the n years experience with them. The tech hiring process is gruelling enough with languages and frameworks we are familiar with, your resume won't even get a glance if you apply for jobs where you don't have that experience. But I think that narrow thinking is to everyone's detriment.
I've seen at least a few Elixir jobs where they explicitly specify that you don't need to have experience specifically with Elixir. Don't know how common it is though, and it's true that finding Elixir work can be difficult to begin with.
I agree, and to be clear I've never really been in a position to worry about hiring anyway. But I would imagine building a medium/large team with varying experience levels (including juniors) would be easier with a more established framework like Rails.
I worked at a well known company running Erlang. Out of maybe 25 people hired to work on software in Erlang, I think two had used it before, and they were hired several years after Erlang became our key enabling technology. I was ahead of many because I remembered seeing the slashdot post when it was open sourced.
If you hire smart and flexible people, and give them a bit of time to learn the syntax, Erlang and I assume Elixir will bend their mind to a new shape, and they'll be fine.
Figuring out what you want the computer to do, and what steps will be needed are much more important than the specific syntax you use.
I would reach for Rails when I'm concerned about finding developers (Elixir devs are fewer and more expensive, generally), or if there are Ruby libraries I want that aren't available in Elixir.