How are declarative distros different than Kickstart?
AFAIK Kickstart is only usable for the initial installation of the OS, but not for subsequent package maintenance upgrades/downgrades.

Unless I miss something (quite possible, I don't have much experience w/ them) the Linux distros managed by configuration management tools require a minimum level of health from the OS (and some package requirements, at least in some cases) to be able to operate those tools, say to perform a rollback. At least NixOS appears to not have such requirement (or rather it is already fullfilled at the OS level). From About NixOS:


  Rollbacks
  
  Because the files of a new configuration don’t overwrite old ones, you
  can (atomically) roll back to a previous configuration. For instance,
  if after a nixos-rebuild switch you discover that you don’t like the
  new configuration, you can just go back:

$ nixos-rebuild switch --rollback

  
  In fact, all old system configurations automatically show up in the
  Grub boot menu. So if the new configuration crashes or doesn’t
  boot properly, you can just roll back by selecting an older
  configuration in the Grub boot menu. Rollbacks are very fast: it
  doesn’t involve lots of files having to be restored from copies.