Occasionally NixOS updates break, and I find myself unable to build a new system until it's fixed upstream. But then I realize I can just revert back to the last stable commit of my config git repository and be on my merry way. Nix really brings some of the best aspects of software development (such as a last known good commit) to configuring your OS.
By the way I can recommend "nh" as an excellent wrapper around the basic system tools. It provides a lot more information on what packages were updated, added and removed.