> Interesting. I just spent a couple of weeks learning Elixir & really liked it. Any disadvantage to doing the backend work in Elixir instead?

I haven't really used Elixir so can't talk from experience. I did look at it but just prefer Erlang syntax. I hear good things about the language, ecosystem and community though. Phoenix seems really well-regarded as a web framework (and indeed, there is/was some integration with Elm).

It uses Erlang's underlying VM (BEAM) so benefits from the stability and concurrency BEAM brings. So I don't see any obvious downsides.