Nix is always doing its own thing. The community is fragmented and there are no enforced packaging conventions. They have a "best practices" page that lists language features you're not supposed to use because they break reproducible builds, aka the whole point of Nix. And they don't even restrict network access by default.

It's also not doing runtime sandboxing at all. Isolated builds protect you from supply chain attacks but not malware or vulns in the actual code, and they don't do anything for closed source apps.

Flatpak runs everything in a container with access limited to what is declared in the manifest and you can restrict it even more with Flatseal. It's not as secure as a Firecracker VM but it works way better than any other package system on Linux when it comes to security and distro independence.

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.

Agreed. With all the improvements in namespacing I’m seeing the potential of all desktop apps being namespaced in a reasonable way, where filesystem access to e.g. /home is controlled through a kernel-hook prompt callback, with something like “always allow for this directory” etc. That’d be so awesome!

Nix doesn't sandbox applications, flatpak can.