(I don't agree that your other assertions are correct, but I'm most interested in this one.)
I'm excited to try Elixir but from what I've read (e.g. https://medium.com/mint-digital/elixir-deployments-on-aws-ee..., http://engineering.pivotal.io/post/how-to-set-up-an-elixir-c...), much of my recent experience in running production apps at scale won't apply to Elixir/Phoenix.
There's a Redis adapter for Phoenix channels that you can use in environments like Heroku where clustering might not be an option.
If you want to use built in Mnesia database on an immutable or cluster, attach block storage and configure it to store to that disk so you can remount when you deploy your immutable image.
Hot reloading is the only one where you NEED mutable servers.