I've been working in both professionally for several years now and talked with many over that time, at conferences and such, and have yet to hear "horror stories", but such stories could definitely be useful as illustrations on how _not_ to build a system in Erlang or Elixir, so you should share examples!
I _have_ seen some projects with pretty terrible code, but that has nothing to do with Erlang/Elixir, and everything to do with the skill level of the team behind the project.