Only issue is with Nix as a long time user, it does not provide a standard way to run your nix builds in some kind of container, there are so many different competing third party systems and some first party systems built in nixpkgs it's self like building oci or docker containers or running systemd containers or even building qcow2 images, each with their own draw back and positives.

More and more I have found myself simply using podman and dockerfiles finding that building nix systems simply gets error prone and annoying and all the containerization systems leave a lot to be desired. Though I have some custom scripts for utilizing btrfs snapshots to create different nix store snap shots for containers or vms running with nix stores with virtiofs. This works quite well but not ready to be released and fiddly when things break and requires BTRFS.

I do agree flatpak and friends are not great though.