In my case, ultimately, paranoia has won. :)
How much do you really trust discord? Or games on steam, or slack, or zoom client?
Flatpak does provide you, out of the box, with a sandbox for whole discord app. Sandbox is built on standard linux stuff like bubblewrap / seccomp / namespaces. This should prevent discord from, for example, accessing anything other than ~/Downloads (rw), ~/Pictures (ro) and so on. Or seeing what other processess are running on my pc. Or snooping in my ~/.ssh.
Some stuff might not work out of box and will require reading app's README or wiki. I imagine stuff like "john is listening to song - artist" would require additional permission or configuration. File sharing could be complicated if, for example, discord could not access my screenshots directory. Webcam, voice and screen sharing work out of box but are protected by "portals" provided by (in my case) KDE.
Discord, for example, does not pollute my /usr or /home. It stays somewhere in /var/lib/flatpak/ and keeps my user files in ~/.var/app/$APPNAME. Steam, Slack, all other "big proprietary apps" also keep their stuff in ~/.var/app/$APPNAME. It's not really a choice for them; flatpak just mounts proper directories in proper places and it works really well.
You can use `nsenter --target $PID --pid --net --mount --ipc` and look around to what kind of access flatpak apps have to your system.
Sorry about preaching, I just like that bit of additional separation from stuff I don't really want to trust or care about.
Take a few minutes to learn how it works then. Flatpak doesn't replace apt, which is a system package manager. Flatpak is for standalone apps, like Discord.
Flatpak provides basic level of sandboxing that ensures non-malicious stupid stuff like how steam had a bug to delete the entire user directory doesn’t actually impact you.
Beyond that, discord probably won’t solve issues with the N package managers if something happens. They’re a lot more likely to solve a Flatpak problem though.
