Immutable can be flexible, just like NixOS is with nix shell and other features I don’t yet know about.
Containers are great but rootless has issues with programs that need capabilties like CAP_NET_RAW, so I also need rootful containers. That’s annoying and is an advantage with nix shell.
I’m not a fan of A/B root, which I believe VanillaOS uses. Also an advantage of NixOS is it’s big repo… On Fedora I had to package some programs myself in copr (tried out a less well-known wayland compositor) On NixOS I had to too, but it’s far simpler without the need to build on someone else’s infrastructure.