My experience with flatpak has been stellar from a technical perspective has been stellar.
Where it currently falls short for me personally is trust. With my distro I am putting my trust into the maintainers, but with flatpak its... random people for most apps?
It is tough when it is not a primary channel of distribution for most devs, but I am optimistic that will change in the future.
Sandboxed just means an app can't reach out to the rest of the OS. What about the information I am entrusting to it to process?
If my browser is a flatpak, it likely has access to most of the information I care about. If I am using a chat app that is a flatpak, it can read my most personal communications. Why do I care if it can read what is in /etc?
Relevant: https://xkcd.com/1200/
You totally missed my point. My point was that a lot of flatpaks are packaged by unknown third parties. I would love it if the devs would package things as flatpaks directly, but that is mostly not the case.