It almost feels like these are two completely distinct worlds, but because they're part of the same language and community, it's difficult to feel proficient or even 'adequate' at times.
I never quite felt this way as a PHP/Ruby/JavaScript developer, as for the most part the challenge seemed to be knowing the right frameworks, new language features, or bundler/build tools to be 'good'.
In the most recent episode of Elixir Outlaws, the topic of overusing OTP/processes/Agents is discussed (https://elixiroutlaws.com/37), and it made me feel a bit better about my lack of experience in this area (and much as the podcast can be on the 'whiny' side of things, it's probably my favorite Elixir-focused one and I highly recommend it! If you're reading this: Hi Chris, Amos and Anna. being a friend of the show one day is on my bucket list :).)
If it's just plain old request/response styles website (I think most of the project falls into this bucket) if you can handle other frameworks like Rails or Python, you can handle the same thing really well after some usage. Some basic knowledge of the language/MVC/Ecto would do.
Though there are a lot of React developers already using React but don't know what they are doing. Using React actually requires a good sense of the ecosystem - which set of libraries to use, which pattern to follow etc. Plus some basic concept like state/props/immutability/HOC would do.