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 haven't tried it but doesn't flatseal let you setup steam's permissions to allow external/additional directories or mounts?
What's stopping steam's access to other directories?
Ah, I haven't heard of flatseal before.
The trick is knowing how to do it. I still haven't fixed my Zoom install to successfully download emojis (which I suspect requires a filesystem permission it doesn't have by default)…
It works when set up with flatseal.