The other issue is that Elixir and Phoenix have become joined at the hip in the same was as Ruby and Rails, and Phoenix didn't click with me at all, coming from the JVM world I really didn't get on with it.
After reading Programming Elixir, Elixir in Action and Programming Phoenix I decided that it wasn't an easy way to make web applications that don't need OTP, and in fact I couldn't imagine a scenario where any web application I would ever write would need OTP (for the whole thing anyway, I can imagine cases where small parts of a larger system could benefit massively from fail fast architecture)
Edit: I would also argue that designing an OTP based system architecture is harder than just writing a non distributed system and then adding in load balancing or fail over.