To be honest, I’m not very familiar with the Nix expression language, so I’m not certain I understand what this is doing. By the looks of it, it’s editing the attribute, and you use anew (nifty tool, thanks for the link btw) to skip duplicates. Seems like a good idea. But, if I understand correctly, that doesn’t handle removing packages, or rolling back to the original version if home-manager fails.
I wasn’t sure myself, so I looked it up, and there have been some posts asking that same question: