It’s worth noting that NixOS and Qubes aim to solve very different problems. NixOS (and Nix itself) tries to improve package/dependency management, allowing for things like rolling back upgrades and flexibly using multiple versions of the same package. Qubes targets sandboxing of individual services/apps, with the goal of preventing lateral movement within an endpoint between applications.

NixOS doesn’t sandbox apps by default (obviously, the user could run all their apps using containers/VMs/etc, but the same is possible on other distorts).