I am on Fedora so the equivalent is COPR.

Flatpaks can be built pretty messy, use outdated runtimes or even entirely outdated dependencies.

It is pretty creepy, I digged down the pyramid of dependencies of OnionShare once and that thing is huge, some projects are archived, some had new releases but it still uses the old versions.

Native packages might not bundle all that in, which means more effort but especially more updated packages.

The sandbox is determined by the packagers, and a mix between "dont make it too loose" and "dont break use cases". For example many big projects without portal support have host permission to access your theoretical SMB shares or external media.

But yes, the bubblewrap sandbox is there, it prevents apps from manipulating the system, the syscalls are a bit restricted via a "badness enumerating" and pretty loose seccomp filter.

This prevents all apps from creating user namespaces, which are like chroots and create a small virtual filesystem for processes. They are used in FF and Chromium for sandboxing. But Firefox also uses seccomp-bpf which works within a flatpak.

If you want a Chromium browser, it should be native. Firefox arguably too, as it gets another layer of sandboxing. But Flatpaks are isolated from the system.

Have a look at bubblejail, which allows to sandbox programs from the OS with bubblewrap, but with a custom filter that can allow user namespaces.