Depends on what you consider "easy"! Nix itself has a pretty high barrier to entry.
Personally I believe systems that start simple (e.g. Alpine) are easier to mess with. Plus you don't have to give up all benefits of declarative configuration; for example apk has a single file (/etc/apk/world) that defines the exact package set that needs to remain installed. You can edit it and run "apk fix", much like you can edit /etc/nixos/configuration.nix and rerun "nixos-rebuild switch". It's not as powerful as Nixos, but power (and complexity) always has a price.
Personally I believe systems that start simple (e.g. Alpine) are easier to mess with. Plus you don't have to give up all benefits of declarative configuration; for example apk has a single file (/etc/apk/world) that defines the exact package set that needs to remain installed. You can edit it and run "apk fix", much like you can edit /etc/nixos/configuration.nix and rerun "nixos-rebuild switch". It's not as powerful as Nixos, but power (and complexity) always has a price.