KDE has the newer QML-based Kirigami and older Qt Widgets frameworks that are not consistent. Widgets apps absolutely look like actual desktop apps (menus, toolbars, dialogs), Kirigami apps look like mobile apps (nav bars, hamburger menus, page-based navigation). There is definitely a visual and functional inconsistency between the two, even if they use the same theme pretty well.

The problem with things from the Linux world is that they never reached the height of commercial desktops to begin with.

If you really enjoy worse Windows XP UX with hamburger menus in recent versions then by all means go ahead, it does function.