That problem is overstated. I've been dozens of custom packages and modules and it's not that much more complex that basic Nix config, and in fact is a huge positive, not negative. How easy can you write config that packages a binary and/or sets up a new service reproducibly? I'd much rather do that on Nix than Ubuntu or Arch for sure.

If you modify a base package, like maybe to apply some minor patch on a systemd executable as an example, is there a way to avoid having to recompile basically all installed packages (for being at least indirect dependencies)?