Many flatpaks are not aware of their sandbox and thus have a bad ux.
E.g. flatpak Steam can’t access SteamLibraries at a non-default location, unless the user manually allows the path through flatseal. The same is true for other similar apps which don’t use the file portal.
Issues like this are unexpected for new users and thus it can be argued that flatpak aren’t a good recommendation for new users. I personally disagree because most flatpak work flawlessly and work everywhere independent of a users distro.