My main issue with Elixir (and maybe this has changed in the last 3 years since I looked at it), is that there isn't a strong open source package libraries available like you see in Rails. People complained that you end up writing boiler plate code for things that in Node or Ruby would just be a package.
I've heard great things about the language and framework though.
I haven't run into many issues where I wished there was a package, but I haven't programmed a web app in a language that had a lot of packages available.
The main close call I had on this project was that there was only one package that supports generating excel spreadsheets, and it's a labor of love by one person. This wasn't a huge issue for me, I had a plan to write csv and then convert if it didn't work, as I didn't need formatting/fancy output. It turned out to work great, so that wasn't necessary.
There was an excellent transactional email library that hooked into the standard templating system, and a few so-so approaches to authentication. I ended up rolling my own on top of a decent crypto library.
For the non-web-speicfic bits I haven't found this to be an issue, Erlang has a whole kitchen sink with OTP (even if like a real kitchen sink it's a bit grungy if you look inside), and I've found plenty of algorithm/data structure-y packages.
I've heard great things about the language and framework though.