I've been using Elixir in production for four years now, in a fairly complex SaaS app, and I can comfortably say I've never had to "learn all the gotchas of BEAM and OTP". IMO Elixir/Phoenix do a really good job of abstracting all of that away from you, while still making it easy enough to tap into their intricacies if needed (e.g. if you want to write a GenServer for something).

Lack of library support was a bit annoying at first. But then I realized that, back when I used to take advantage of existing libraries in Ruby/Rails, in the vast majority of situations I was just utilizing a small portion of those libraries anyway. It ended up easy enough to write my own code for those features.

I have written Rails for 8 years and Elixir for 3. Phoenix is essentially what Rails should have been.

It has the upsides of Rails without the connected antipatterns, native technical debt.

I can also confirm that there isn't a need to learn BEAM gotchas (which i did learn). One of my ex colleagues is a junior dev who has been using elixir for 3 years now and didn't need to use those special aspects of BEAM.

As a Ruby expert, Elixir is substantially easier, learning BEAM is a joke (and very instructive) compared to learning the entirety of the Ruby object model.

I can't think of reasons to use Rails nowdays. If you are in the CRUD apps market, use Postgrest or similar products. As for the rest sure, choose any language you want, the framework is way less relevant (arguably: detrimental) at that point.

The industry has proven extensively that for non-trivial projects, Rails is damaging due to ActiveRecord.