I would not use Elixir when… I don’t have the choice to (such as native mobile apps)
I would also not use Elixir for very basic sites or when other languages excel in those areas (such as image processing) but still use Elixir as the ‘brains’/core of the app.
I like to stick to one language if possible, so hopefully, when I am fully acquainted with it, I will use Elixir most of the time.