The general issues people have are around Application env, compiled System variables from build box in their configs, people forgetting that module attributes are compiled, and those sorts of issues. Libraries have begun moving away from Application env per library guidelines, and have begin moving to init/2 callbacks instead, which alleviates a lot of those issues, but there are a lot of straggler libraries that will not change for a while. So you have to do funky things like play with your Application env in your application.ex file. You had that weird system tuple thing that some libraries supported, and some that didn't, and then things like Confex coming up to meet in the middle. There isn't a nice story for getting secrets from something like Vault, for example.

Releases are not mainline yet (soon!) and are kind of off to the side in a semi-officially supported way (bitwalker is amazing!). There are guides for this, but not the same type of guides as would exist if it were in the core repos. Then you have hot code reloading, which exists, but is usually said to be shied away from generally, so there's even fewer guides about all the gotchas that come with those sorts of systems. Some people even run mix phx.server on their prod boxes!

I LOVE this language, and all these problems have solutions and workarounds, but one can't tell the story without being honest about the current release/deployment quirks. Things are definitely better than they were a few months ago, and much better than a few months before that. But they're not quite at the level they need to be for something that's supposed to be so fundamental to your application as its configuration and release. They'll get there.

Oh I ran into some of those issues too. Getting runtime configuration variables can be a bit of a pain. But on the plus side, I added a tuple (MFA) option in a PR for a lib and Jose Valim merged it a few days later. Really helpful core team!