What specifically to you about cross-platform UI means bad job? I.e. what functionality does Windows specifically support that must work when running the application on Windows that Gnome, for example doesn't? (Or any other GUI/OS comparison)

A lot of it boils down to native widget look and feel. It's pretty silly in reality because there are multiple possible options for what is native on Windows (Win32/WinForms, WPF, UWP, WinUI, ...) and Linux (Qt, GTK, ...), so native apps will still be inconsistent. It's probably just an Apple thing really, because every other platform is already a big mix of different UI styles.