The sweet spots for me are:
1. It is much easier to trace things through the entire Phoenix stack than it is in Rails. It is also much easier to add things to the Phoenix stack using plugs.
2. Elixir is concurrent, whereas Ruby is not, so when performing long-running processes, you can just do them in Elixir/Phoenix without having to rely on workarounds like Sidekiq, Resque, RabbitMQ, etc.
3. Writing multi-threaded applications is much easier in Elixir than many (if not all) OO languages.
4. Pattern-matching for variables and functions, and binary pattern-matching for parsing text.
5. Mix.
6. BEAM, OTP, and supervision trees.