Icon design and name guidelines are too strict. Many apps already on the store break them.
E.g. the 20 characters limit is to small for even two long words.
Or not allowing PascalCase, disallows many app names like LibreOffice.
And app icons have to be "In line with contemporary styles", which is currently flat and simple design. Contemporary styles change all the time. If an app wants to look old, why not let them?
Edit: Generally these guidelines are good and will improve the browsing experience on flathub. Hopefully they'll loosen some of those icon and naming restrictions.
Opinions on the flathub metadata guidelines? ( docs.flathub.org )
I do think that as a store it should have some guidelines on quality, but not of the application icon's styling