Every Flatpak vendor
So who’s that? Flathub and Fedora, the latter of who automate the Flatpak builds from distro packages anyway.
If you’re using a smaller distro which is not backed by a huge security team then this is probably an advantage of using Flatpak, not a negative.
Yes. All Flatpak apps can be used on any distro.
I’m using the Fedora Flatpak Firefox on Debian, because Fedora’s Flatpak runtime supports Kerberos authentication, the Flathub runtime doesn’t.