When I was packaging Flatpaks, the greatest downside is
No built in package manager
There is a repo with shared dependencies, but it is very few. So needs to package all the dependencies... So, I personally am not interested in packaging for flatpak other than in very rare occasions... Nix and Guix are definitely better solutions (except the isolation aspect, which is not a feature, you need to do it manually), and one can use at many distros; Nix even on MacOS!
Well; darwin users, just as linux users, should also work on making packages available to their platforms as Nix is still in its adoption phase. There are many already. IIRC I, who never use MacOS, made some effort into making 1 or 2 packages (likely more) to build on darwin.