Yes, there's one reason not to use Elixir that I know of: [Edit: It appears I might be wrong on this.]

It doesn't fully cover the erlang syntax. There are a few things you can do in erlang that you can't in elixir.

As I understand it, this will be resolved relatively soon, in an upcoming release, possibly only in a matter of months.

Elixir and erlang can be mixed pretty easily, so there's no harm in trying a module in elixir just to see how you like it. Underneath, it's "erlang all the way down", so you're unlikely to run into problems.