I always use Flatpaks when available, I have been using it for about 1~2 years and honestly, I haven't found any issues that are deal breakers, mostly some missing storage permissions, but KDE makes this easy to deal with. I know some apps have some issues, but the biggest one that I had is that Steam Flatpak still requires Steam-Devices to be installed as a package, but that's more to do with the way Steam Input works.
The only issue that I have is that uninstalling Flatpaks should present an option to delete the app data.
A family member always downloaded and installed APKs from the internet and didn't know about the Play Store cause that is how you did things on Windows.
The person is question has low tech literacy, and they were doing this for years