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?
I see that fragmentation of runtimes is a problem. If all apps would simply use the same runtime, and a modern one, and there was a package manager that installs the missing dependencies, that would be nice.
The diskspace is a true problem too, just because of the fragmented runtimes.
But Distros are fragmented too. If simply everyone could unify, at least a bit, instead of at least 5 different big Distros competing, every app could just work. But thats not the case, so Flatpaks often work best, and maany packages are either only .deb, .rpm or even only on Arch