They're fantastic and I hope the future is bright for them but I've had to pick Rails for a few projects over the last few months because the library support is so much more established. The more I get to use Phoenix, the more I am convinced it is the most well designed web framework there is, so I hope to use it a lot going forward.
It took me a couple of hours to build my first demo with it, have a look: https://youtu.be/tTPH4DyUaUk
and I didn't have to write any JavaScript for it.
Elixir/Erlang's emphasis on uptime/fault-tolerance/scalability/concurrency (admittedly at the cost of performance and the abstraction of a VM) is perfectly suited to web/mobile use-cases.
On paper, Pheonix should be the default choice for large and small web/mobile MVC applications. I think the question of if that becomes the case is directly tied to adoption of Elixir.
The performance comparison matters if you start comparing nginx, and maybe even the Go/Rust frameworks, but it absolutely blows Rails out of the water for performance without sacrificing any of the high-level abstraction goodness that makes Rails appealing. No reason not to move to Elixir unless you cant hack functional programming IMO
