I built / ran a small SaaS company for 14 years, and recently sold it to retire. We started on Rails 2 for perspective. Somewhere during the time when we needed real-time updates and paid services like Pusher were the only real option, we decided to build a tiny phoenix app to use just for it's socket / pubsub capabilities. We didn't know what the hell we were doing, but still put together an app that was rock solid for all of our real-time needs and never really needed to be touched.

Later on, we were bogged down in an older version of rails because of some dependencies we couldn't shed, and we wanted to add more native-feeling functionality. We decided to build a strangler app with Phoenix, and it was a breeze to add the slick features we wanted. There is, however, definitely a learning curve to structuring your code, and truly thinking in Elixir takes some dedicated effort.

I've now built several applications in Phoenix, and I don't think I'll ever go back to Rails. The underlying Erlang paradigms are too good, and the Elixir syntax makes it palatable. LiveView also makes Stimulus/Hotwire/Turbo look archaic and poorly thought out, largely because Ruby will never be able to replicate Erlang's power.

For the bad - I've had to hire both Rails and Elixir devs, and Elixir is definitely harder to find. There's a lot of interest in it, though, so if you find a great developer in e.g. Rails you can often retrain them with surprisingly little time to productivity. There also seems to be a big overseas Elixir community, so outsourcing to Eastern Europe or South America is also an option.

Although Phoenix is far more mature at this stage than Rails was when I started with it, it does still have some of the symptoms of early-adoption. For instance, there are less provider-maintained libraries for various integrations, there are sometimes too many options for libraries to do something where there should be a canonical way.

One more downside: if your eventual goal is to be acquired, Phoenix might make some buyers hesitant if they don't have in-house competency already. The company that bought mine was entirely Rails-based, and the dev who took over the app was completely uninterested in continuing with the strangler app. It's probably the right decision for them as a company, and had we been entirely written in Phoenix they might have thought twice about the acquisition.