My only real complaint with Elixir (and this covers Phoenix and Live View too) is that things tend to get announced WAY before they're anywhere near being production ready, so you may find yourself postponing building anything because you're waiting for the announced thing to be ready to use.
For example Live View was talked about in the later months of 2018 but there's still some things missing today that make it pretty sketchy to really use it in a non-trivial production app, and the docs leave much to be desired. Things like LV based file uploads as being part of the library was in the "near future" or "on the horizon" a year ago, etc..
I only spent a weekend using it for a project to play with about 6 weeks ago and ended up uncovering some bugs and even some feature requests that quickly ended up making its way into the master branch of LV after reporting them. But these are for things that seemed very basic to me in the sense that if the folks who were developing it were using it in larger production apps, there's no way those things wouldn't have made it into the library sooner.
In other words, I don't feel comfortable or confident at all to use it in production because I'm afraid if I spend some real time working with it, I'm going to end up being patient zero for so many bugs and edge cases. I want to focus on building my apps and trust the libraries I use, not be a pioneer (software is already difficult enough without bugs!).
On the flip side, Rails is a champion here and IMO it's very much why it's so popular and will remain popular 5 or even 10 years from now. It's because most of the things in Rails come from tech focused businesses using it to drive their services and the core team also use it to drive their business. Everything feels like it's really battle hardened, production ready when features are talked about and was carefully designed based on months of real world usage.
With Elixir, Phoenix and LV that feeling isn't there -- at least not to me. It sort of kind of feels like the libraries are being built from a theoretical point of view. I mean, I know the creators of those libraries do consulting work, but it's super behind closed doors. There's never any talk about what drives the development of practical features and it's kind of a bummer to get glimpses of things on the horizon, but then years later they aren't ready for production.
That's the thing that worries me about the future of the ecosystem. I also think it's partly why it hasn't exploded in popularity. It's a nice ecosystem for sure, but it's missing critical components for it to be adopted by the masses.
That and I think generally speaking the language is very hard to pick up. I know I struggle hard with Elixir with ~20 years of general dev experience across half a dozen non-functional languages before touching Elixir. I still feel like I have trouble reading Elixir code I've written 2 months ago when it uses some "real" functional bits that I had to ask for help on.
I often feel like I hit 100% dead ends and have to ask another human for help. I don't think I would be using the language if it wasn't for the IRC / Slack channels. There's some folks going above and beyond to help people out there, including Jose himself (the creator of Elixir).
With Python and Ruby I rarely encounter situations where I had to ask for help like this. Google always had the answer for everything I couldn't figure out based on tinkering and reading the docs.
It's not just due to less blog posts existing for Elixir / Phoenix too. It's the language overall. I want to love it, but for whatever reason my brain won't accept it naturally. I get hung up so frequently. I know this is just my brain being an idiot, but I do wonder if anyone else feels the same.
My gut tells me yes because otherwise the language would already be one of the most popular choices for building web apps. There's a lot of great things surrounding the language.