The thing is that Flatpak failed at providing good sandboxing. Some flatpaks are well made such as as Firefox, Mozilla really put effort into getting something well packaged but unfortunately that's not the standard. Most of them are packaged like shit or for the better ones they are not as well packages as the distro official packages that have stronger quality assurance. Of course many apps only have debian or arch support and flatpaks does bring wider Linux based OS support which is cool and I do run some flatpaks on my devices.

But I hope we will have something better that will properly be sandboxed and have stronger requirements for packager to offer a global GNU/Linux publishing experience regardless of the distro, even if it comes with some drawbacks like size.

I am not hating on flatpaks but I think people over estimate the sandbox aspect and simply don't see how poorly most of them are packaged.