1. Hiring. Generally speaking, finding experienced Elixir engineers is not cheap nor easy. Also, chances of finding someone willing to work in an office is even more slim (very remote heavy talent pool)...
2. Deployment/monitoring. Shipping your first Elixir application to production will likely be one giant act of frustration if you're used to Ruby or other scripting languages. Compile time vs runtime settings and environment variables are a HUGE gotcha. Also, as fate would have it, BEAM is more like an operating system than a simple runtime and thinking you can toss it into container and be off to the races is a recipe for disaster. Hope you didn't want to use distributed Erlang... (though this has gotten better recently, tbh). A lot of common monitoring tools and methodologies have to be thrown out the window because of how BEAM works internally (there are great ones to accomplish most anything, but its just more to learn).
3. Distributed systems, make for distributed problems. Yes, Erlang/Elixir and BEAM give you some fucking amazing tools to build distributed systems... that doesn't make building distributed systems themselves "easy" or "quick."
4. Performance. Erlang/Elixir will probably take a hot-fast shit on most scripting languages when it comes to network requests; however, if you need to do any long-running number crunching or complex string manipulation, you'd be better off almost anywhere else. This could also be categorized as "use the right tool for the job, dummy."
5. Learning curve is real and vastly underestimated. You aren't just learning a language, you're sort of embarking on a quest to learn a new paradigm. Erlang/Elixir, BEAM, and OTP (I'm going to refer to them as the "Triforce") were designed together to solve a specific set of problems. The Triforce has been in-use, 24/7, 365 days a year, for 30 years, in real, critical, production systems. If you ever wanted to learn the "Actor" pattern, BEAM is the most metal implementation I know of... Also, really sit down and make sure you understand it's all "non-blocking" thanks to the scheduler being able to pre-empt processes.
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With regards to 3rd party libraries, I find in Elixir and Erlang, I need and use way less of them... BEAM and OTP give you soooooooo much it's kind of absurd how little you end up missing or wanting.
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Lastly, while a lot of the success stories here are great to read, a lot of them have bad code, or things that are just silly to the trained eye. And that's totally okay, the authors are learning, we all have do it, but I'd probably not recommend the code as study material. In general, try and find examples from people who have seem to have been using it for a few years.
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Addendum (via edit):
FWIW, I've been writing Elixir for about six years now (just checked some commits), and have been employed full-time writing Elixir for five of those. I wrote Ruby before that, and have never looked back... If you want my number one selling point to any Rubyist :trollface: (but also very real), is in Elixir a "horrible, slow, wretched monolithic test suite" still will take less than 90 seconds to run.