To people who keep picking a specific app to "demonstrate" that the Flatpak sandbox is broken: This is called cherry picking.
Nobody claimed the Flatpak sandbox was a magical solution. For certain things apps need upstream support, but hey everyone benefits from it!
• We get proper and secure desktop APIs, yes also non Flatpak
• Apps become more secure over time
• More control to user over permissions and data
• No random files in ~