Yes. Almost all of them, actually. By "build" I mean the whole package installation process, including moving the compiled files into their final destinations.

Yes. Almost all of them, actually. By "build" I mean the whole package installation process, including moving the compiled files into their final destinations. 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.