I mean why would you be fully against flatpak? I use NixOS without it and always packaged natively on Arch, but especially when upstream offers flatpak, it makes sense to enable it. Keeps the user-facing programs up to date and somewhat sandboxed while you can have a stable release beneath it. Especially if the system’s actual users aren’t that tech-savvy.

Stuff on unstable tends to break, especially electron-dependent derivations. Stable doesn’t always have the latest and greatest. Flatpak seems like a good compromise for desktop applications in some cases.