I think my biggest issue with flatpak is still the attitude it and its proponents have towards disk space. The idea that space usage isn't really a concern, and it's okay to require users to download 2GB of platform libraries that they probably already have to install their first flatpak boggles my mind. Because of this, it always seemed hostile towards distro packagers. (If you just used more flatpaks, the bloat will be amortized!)