There's a lot that I like about Erlang the language, but using it in production left a sour taste in my mouth due to difficult-to-find documentation, difficulty in testing the code, and (especially) the misery that is mnesia.

It was a poor fit with our cloud-based infrastructure, and once the mnesia database began corrupting itself weekly, requiring a full rebuild, it was an easy decision to move to another solution for what we were using it for.