I don't personally take issue with Flatpak (though I don't really like Snap), in fact I would likely publish my own applications as Flatpak if I needed to, I just wanted to answer the question of the grand parent.
I can't really speak for the Ubuntu PPA, but the openSUSE repository is the big third party software repository that almost everyone uses anyway. I'm not sure if it's quite accurate to call it the equivalent of the AUR, but it should be similarly widespread.