The package manager could just flock() a specific file before making any changes.
By atomic transaction, we mean simply this: either all of the changes happen in the repository, or none of them happens. Subversion tries to retain this atomicity in the face of program crashes, system crashes, network problems, and other users' actions.
Does Nix mean the same thing? Is it atomic in the face of program or system crashes?
I suppose there is still a window in which an error could leave the system in an inconsistent state, but it's pretty narrow, and the error would have to be a kernel panic or something like that.
If you're upgrading an existing package, and that involves overwriting one binary in /usr/bin and another in /usr/lib, a crash in the middle of that process leaves the package partially upgraded, and very likely broken.
Nix never overwrites an existing package installation. Instead, it installs the new version in a separate directory, then overwrites a symlink that pointed to the old version with a symlink pointing to the new version.
1) If you install a bad grub, you cannot rollback it using nix. That means, not a wrong grub line, but if grub itself is broken.
2) Anything that is not under nix control, the data: like desktop configurations, databases, ecc.