Elixir and concurrency
It seems to me that people in the Elixir community worry quite a lot about data races.
And, I guess with Elixir you might bump into those kind of issues quite more often than in other platforms, as you are doing things concurrently far more often.
It feels hard to push forward with Elixir, but I guess it's just because the platform forces you to think about them, while in other platforms you don't even realize you have these issues.