I’ve been involved with Linux for a long time, and Flatpak almost seems too good to be true:
Just install any app on any distro, isolated from the base system and with granular rights management. I’ve just set up my first flatpak-centric system and didn’t notice any issues with it at all, apart from a 1-second waiting time before an app is launched.
What’s your long-term experience?
Notice any annoying bugs or instabilities? Do apps crash a lot? Disappear from Flathub or are unmaintained? Do you often have issues with apps that don’t integrate well with your native system? Are important apps missing?
Flatpak also installs dependencies, which are shared across Flatpak apps - So yea, the initial download sucks, but native apps actually do the same thing (you just already have the dependencies downloaded).
Also for disk space - If your fs has compression by default, a lot of it gets saved
File a bug report upstream on a per-app basis