Home manager and flakes is something optional in today’s NixOS. I don’t see any obvious use for a single-user home system. Of course, many people go the flakes-setup route. NixOS has many customization options, up to and including docker installation.
It doesn’t make any difference. Gentoo and NixOS have the same concept. They are holistic systems not designed for multiple permanent changes. I’ve used Gentoo, it’s as much fun as building everything from FreeBSD ports. But some users install the OS to get work done, not to constantly tinker with the system, so now I choose NixOS.