In general, functional programming in Scala tends to be more FP, with code tending to be more pure than in Clojure and I have no experience with Elixir, but I have some experience with Erlang and FP code in Scala tends to be held at a higher standard than in Erlang.
Of course, you've picked 2 dynamic languages as examples and FP in dynamic languages is different than that practiced in static languages like Haskell or Scala. LISP developers for example don't think so much about monads or other abstractions with mathematical foundations, because LISP developers tend to work around such needs by doing macros (which then have composability problems) or by bending the rules a little, or in other words I've seen no LISP to make a serious attempt at reasoning about I/O in a pure way.
And IMO code in static languages tends to be more pure because of the types, because by having an expressive type system, the developers then want that type system to explain everything. Or in other words, dynamic languages are cool for your day job, but if you want to actually feel what FP is all about, you're better off going for a static languages like Haskell, or even Scala or OCaml.
You cannot get away with these things as easily in the Erlang VM (and thus Elixir). I agree with your assessment that Clojure with its macros is an ugly hack, and requires discipline to get right.
Having said that, Haskell also takes a lot of discipline to get right (no lazy I/O, for example), and allows you to get away with ugly things as well (unsafePerformIO). The type system makes it a lot easier to get right though (or in other words, more difficult to do the wrong thing).
