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.

Maybe I am vastly underestimating it, but I could imagine this being buildable atop of nix quite straightforward?