Earlier quoted context omitted.
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 a…
Maybe I am vastly underestimating it, but I could imagine this being buildable atop of nix quite straightforward?
GUI apps on NixOS rarely care about sandboxing so while you can do it yourself i bet almost noone does. Flatpak is trying to give you basic protections out of the box.