edit: I don't know the answer to your question. What I do know is that it's really hard to get indentation right in Emacs for languages with significant whitespace where new lines are the only marker, and I assume the same is true for TextMate (which Atom piggybacks on for indentation). I'm not an expert on emacs, emacs lisp or creating language modes for emacs, but I do have a decent amount of experience[1]. So when I say I'm sympathetic I mean it down in my bones.
But still, if an editor can't get auto-indentation right, it's a deal-breaker.
[1] https://github.com/elixir-lang/emacs-elixir/blob/master/elix...