I thought the same thing too. A couple responses:
1) The Elixir community is still in its infancy and the community is growing rapidly and flourishing. If the concern is there isn't an equivalent Elixir library for X gem, it's only a matter of time before it is. So please join!
2) I attended the Phoenix training at ElixirConf this week and an easy answer to registration and auth is "hey, write a plug". Once you dive into how straightforward plugs (1) and what (init/1, call/2) do, it's really simple.
You will appreciate the explicit nature of plugs compared to say, Rack middleware...
I'm rewriting a Rails app in Phoenix and overall, I've found it to be a really pleasant experience.
(1) http://hexdocs.pm/plug/extra-readme.html