A lot of what's great about Phoenix is simply that it builds on top of Elixir, which is already great and which itself builds on top of Erlang/OTP, which is, too, already great; it's the "standing on the shoulders of giants" concept. The overlap between Phoenix devs and Elixir devs helps, too.

Phoenix ain't quite my cup of tea for the same reason Rails ain't my cup of tea: it's big, and it does little to hide that bigness. It's why, when I was doing web development on Elixir, I ended up gravitating to alternative frameworks (like Sugar - which, as a disclaimer, I'm a very inactive maintainer of), for the same reasons I tended to prefer Sinatra or Padrino over Rails. For most people, though, "bigness" ain't really a problem - and Phoenix does a really good job of being "Rails but actually good". At the end of the day, it's a solid choice that probably won't bite you in the ass, and that's what matters.