Rails has more mindshare, it's easier to hire for, there are more tutorials etc to help you when you get stuck, and Ruby has a more mature ecosystem of libraries/plugins than Elixir has.
I'd still pick Phoenix over Rails any day of the week, but if I had to make the case for Rails, that would be it.
I can't think of anything specific that was a huge problem. But when you're integrating with a third-party tool (e.g. for analytics, error reporting, email delivery, anything really), it's very common to see that they provide a Ruby SDK but not an Elixir SDK. Or if there's an Elixir SDK, it's something unofficial (and possibly unmaintained) from a third party, while the Ruby SDK is something in-house that has official support.
Most of the time these SDKs are just thin wrappers around their JSON API, so it's easy to build your own thing in Elixir anyway. And LLMs make it really easy. But it's something I've experienced pretty often.
I enjoy Ruby but activerecord is a mess and the language is slow and lacks real time functionality.