For serious apps it's impossible to escape reading Erlang or Erlang documentation. Many functions and libraries simply aren't available for Elixir or are partial (see `opentelemetry_*` family as an example). Deep debugging is almost exclusively done with Erlang functions.

I'd even say that the more serious/critical application becomes the more weight shifts toward Erlang. Personally I'd go with Erlang-first, but only because I've accumulated thousands paper cuts from Elixir.

For starters Elixir has much more palatable syntax, though.