My current opinion on this matter is that I avoid doing anything in releases.exs if I can help it. The reason is that this is a free-form code which runs only in production, which means that it’s not easily (if at all) testable on CI. I’m not confident with writing such code, and so I like to keep as much of config as possible outside of it. Providing config during app boot has a few shortcomings, but on the plus side it brings production and dev/test much closer to each other, and so I strive to have as much config as I can in the app start.
