But you simply don’t have to worry about it. You test it on the next LTS release of nixpkgs and that’s it. Your user may download a bit more data, but I found that nix is lightyears faster then fedora’s build tool, and on par with arch, while not polluting the whole filesystem with random files. I don’t see where would you need more maintainance.
In a tightly controlled environment inside a company where the update mechanisms are preconfigured / enforced, I'd go with nix model. When distributing an app to people on the internet, I'd choose flatpak model.