Elixir is modern tooling on top of the BEAM / Erlang Virtual Machine. This is tech that has been running Telecom systems, companies like Whatsapp, Discord, Klarna, and software like RabbitMQ, etc for decades. The core libraries are very active and well maintained.
I tend to prioritize "capabilities to build compelling systems" which, to me, means getting to production quickly so user feedback / product-market-fit tests can occur, but then also scaling a team to maintain the application over time because once you've found a good fit its time to grow.
To me stateless CRUD isn't compelling. Its too easy - if your app just provides a GUI in front of a database for Create/Read/Update/Delete operations your app is a database not an app and if all it takes for someone to clone your app is 5 minutes of CRUD generators, then that's not exactly a defensible position to build a business on.
Phoenix like other web frameworks provides generators and quick paths for your usual Web + Database applications. Run some commands and you've got a web app with database persistence. Everyone can do this nowadays (Django, Rails, etc) What Elixir and Phoenix does better than anyone else is distribution, concurrency, fault-tolerance, and real time streaming. These capabilities are possible because of deep details on the virtual machine level and become trouble for cooperatively scheduled runtimes elsewhere (e.g. Python, JS, Java, Ruby, etc).
If you're building something with: websockets, collaboration, chat, i.e. anything real time between many users - save yourself the trouble and look into an Elixir/Erlang solution like Phoenix/Liveview/Channels/Presence.
On the human side Elixir has top tier documentation (example: https://hexdocs.pm/phoenix/channels.html#content) - we've found it doesn't take long for developers with at least a few years under their belt to pick up the language/framework and be productive. You don't have to know everything - just build some familiarity of where to look and you'll figure it out. Its very important that this human side be scalable as well, and that means developer experience, documentation, on-boarding, etc.
I've been using Elixir/Phoenix every day since 2017 to build apps, glue systems together, rebuild Ruby/Rails/PHP/Python apps that don't scale anymore and in general just ship features on time consistently.
The thing that I'd toss in as an "it depends" on the technical scaling side is Postgres - its a boring single server relational database that just works but is still a single server architecture made for spinning disk persistence. SSD/NVME/Persistent Memory and more are new foundations for persistence which means new databases on the new foundations may warrant attention soon. 99% of the time just use Postgres/MySQL though. If you're not sure just use Postgres. For many apps in finance or with stringent auditability that are log-centric using a time series and/or event sourcing database makes sense for the projection / read model flexibility. That said you can use Postgres for that so there's no good excuses for not using it.
I'd say the biggest disadvantage/con to using Elixir/Erlang is that its more of an expert's tool-set. These are sharp tools made for engineers that make sense to engineers with experience to know they want fault tolerance and concurrency. What this means is that a newbie developer might not know what they don't know they want to know when building an app the first time. This caveat is probably true with any tech stack, but there will be a lot to learn about the backend because there's more possibilities on this backend stack. These backends-as-a-service (Firebase, Hasura, etc) will get things rolling quickly but in my experience all the real juicy core problems of your business will end up in backend code and when that happens you'd better hope those quick solutions don't get in the way.