NixOS is incredible, but it has a steep learning curve you gotta be prepared for. I've been using it for about 5-6 years and I believe it's worth the price of admission.

I rarely have the need to rollback to a prior build , but the functionality is there when I need it. I use the master branch for maximum bleesing-egdeness, but it's possible to use the stable branch for the base system and use the unstable branch for specific packages.

As for system backups, all you need to keep is your data and NixOS configuration. The configuration makes it easier to get your system back when your SSD suddenly dies or when you want to migrate to another PC.