I've never had a single flatpak program work properly. FP definitely is a super tiny bit better than snap, but they both just absolutely not work for me in any program I ever used it with
I actually understand your frustration and I don't know why you're being downvoted. I can never use flatpaks. They're actually very bad. They're definitely better than snaps, but they're terrible.
Not OP, but what annoys me are permission issues. I installed a program that couldn't access a specific folder and it took me quite a while to find out that this is not an issue of the app, but the flatpak didn't have this directory in its whitelist.
Also, when one flatpak should interact with another flatpak, it gets messy (e.g. steam/mangohud).
That's fair but it's also one of the biggest selling points for me. The isolation it provides is one of the reasons I'd rather ran applications in Flatpak (if possible).
I love the security aspects and it's about time that desktop apps cannot access all your files & apps.
But the implementation is IMO improveable. Maybe by the makers of flatpak, maybe by the app developers. A simple Error message like this one would have helped:
Error: Cannot access directory /data, please grant the permission by following these steps (...)
Maybe by the makers of flatpak, maybe by the app developers
This falls on the app developers. They're supposed to be using something called XDG desktop portals. It opens a filepicker window on the host, the user selects which file they want, and that specific file gets passed through into the sandbox automatically, no permissions needed. Though it's not perfect either -- AFAIK there's no way to pass though a directory using XDG portal, and drag-and-drop is broken (at least on X11). For command-line apps (tho it's not really what flatpak was designed for), the equivalent is the --file-forwarding option.
Permissions are very vague (if that's the right word. Sorry non-native English speaker). Like you start having issues with an app then hours later you find out it has no access. That happened to me with steam when I was trying to use my second SSD to save my games and the thing would never work. It took me hours to find out I needed to give it permission. And for that I had to download flatseal. There should be a warning or something I don't know.
Storage. They take too much space
Theming. They (in my case) would never respect my system theme. They just have their own mind and don't care what theme the system has. You have to use flatseal again to give them some kind of permission, and that I still don't know how to do. And if it's a system theme (like one of those system wide themes, gets even more complicated).
AUR saved me. Now I don't ever touch flatpaks. To clarify, I still appreciate all the hard work being done on them, they're just not for me.
EDIT: all the above is a major turn off for new users. I know they're all user error (except the storage maybe), but a new user is already probably flustered, and they don't need more confusion.