It is an immutable distro, altough it isn't image-based like Fedora's rpm-ostree.
NixOS basically replaces Ansible because the Nix package manager achieves the same goals already (configuration, deployment, ...).
But I agree, the work necessary to put into this non-standard distro makes it hard to recommend for a casual user.
Agreed. I would have like Ubuntu to come with flatpak, but snap exists for longer than flatpak and has additional use cases. Snap allows to do app packaging and even the rest of the system. Fedora uses rpm-ostree + flatpak instead.