Describe situations when flatpak is NOT recommended/cons of flatpak

[pktiuk]

I think it would be good to add here some reasons to NOT use flatpak or at least show examples of apps that should not be packaged. It would help clarify that flatpak is not the silver bullet for everything and help make conscious decisions about packaging specific applications.

https://docs.flatpak.org/en/latest/introduction.html#reasons-to-use-flatpak