Flatpak's sandbox environment is something most people unfamiliar with Linux don't know about. For a distribution such as ZorinOS, aimed at this particular audience, this has a massive impact. You can take a look at the amount of issues raised in this forum that are caused by this exact reason (packages installed as Flatpak by default). Sometimes this isn't a problem, depending on the package itself, and the specific use case. This hit-or-miss situation is the opposite of reliable.
This is technically true, with a twist. After a while you might end up with packages that require different runtime environments e.g.: Gnome 43, Gnome 44, Gnome 45, etc. that need to be installed and maintained separately. This in turns results in double, triple or even quadrupled the amount of dependencies needed.
What's more, Flatpaks that have an update available but that depend on different runtime version will not be automatically updated: you need to manually uninstall and then re-install to the newer version.
I don't know about the security claim on this sentence and in fact I'd like to know more, but restarting the system after a Firefox update is completely untrue.
EDIT:
In the second screenshot of this post you can see what I mean about quadruplicating dependencies (note the multiple gnome platform versions)
And the huge size difference: