I don't remember any changes in the language, but I did encounter packages that had their configuration renamed or changed in a way that broke my config.

Basically you pin to a version of NixOS, and opting into the next version is fully optional. So if you change your NixOS version, and you run nixos-rebuild, and stuff doesn't work, then you can just undo your change. NixOS even keeps a backup of itself ready, so say for some reason you update but your new version doesn't boot, you can simply boot into your previous working version.