My problem with all three is that trust, security, quality comes from package to package. There is no standard, and packages are isolated from each other. If there is an issue between multiple apps, developers just start pointing at each other. With distro like opensuse I know everything is tested properly including security bug coverage and package interoperability. I can even check it myself at openqa.opensuse.org
With flatpaks I am at mercy of each developer not being lazy and well informed about all current issues.