- Completely FOSS stack
- Uses multiple repositories (no lock-in)
- Everything is sandboxed with Bubblewrap
- Fine-grained permission control that offers more than iOS: control whether apps can access the network, which directories an app can access, if it can print, and even whether or not it can access PulseAudio.
- Cross-platform: runtimes are OCI container images and can be targeted on any distro that supports Flatpak (which is almost all of them).
It's gained adoption from a number of recognizable FOSS and proprietary names: Zoom, Spotify, Steam, Firefox, VLC, Discord, Libreoffice, Skype, Inkscape, both Minecraft and Minetest, Microsoft Teams, Krita, IntelliJ IDEs (both Community and Professional), and Blender are available as Flatpaks through Flathub.
GNOME and KDE release almost all their apps as Flatpaks through the `gnome` and `kdeapps` Flatpak repos, and copy them over to Flathub when they're confident that Flatpak-ing didn't introduce any bugs.
It would be nice if Apple would let packages signed by the same key share versioned libraries between them, but I suspect relatively few developers would be able to take advantage of that. Maybe only google and microsoft, to a rough order of approximation.