Flatpak is missing even rudimentary sandbox features. Apps that don’t have an easy sandbox escape are rare. And this is all useless when it’s the app developer who decides the app permissions because most users won’t tinker with every Flatpak they install.
Flatpaks are NOT any more secure than system packages, and their “sandboxing” should not be trusted to accomplish anything.
You can’t ban a certain kind of content on an open source decentralized platform anyway.