[nh2]

Adding some data here:

Total size of installation is roughly comparable between NixOS and, say, Ubuntu.

My laptop's Nix closure of 1 generation is 33 GB. My desktop Ubuntu has 27 GB (20 GB /usr + 7 GB in /var, where snaps and flatpaks are stored).

Indeed the disk usage of Nix comes from multiple generations. Every time there is a new version of glibc, gcc, or anything that "the world" depends on, it's another 33 GB download. Storing the old generation is entirely optional. The maximum disk space needed is 2 generations.

Updating Ubuntu to a new LTS version almost always costs me multiple hours, caused by interleaved questions on how to merge changed config files in /etc (which unfortunately one cannot seem to batch), apt installation being rather slow, and during recent years, the update generally breaking in some way that requires a major investigation (e.g. the updater itself dies, or afterwards I have no graphics). On NixOS, these problems do not exist, and the time to update is usually < 30 minutes.