I'm pretty strongly on the Elixir train, but here's where I see its greatest weakness:

On many platforms, the best practice is to have stateless application servers, with separate stateful data stores. On the other hand, the big advantages of Erlang/Elixir are in managing state along with your application logic.

In a vacuum, that can be a big win. But I'm worried that the advantages are not enough to overcome the weight of tooling, services, and best practices built around the stateless model. For example, Heroku explicitly tells you to avoid application state, and Docker is built around a destroy-and-rebuild ethos.

For my project, Elixir/Phoenix is great as a "better Rails", but I know I'm only scratching the surface of the platform's strengths.