Personally, I find that any Windows application that is remotely polished will have its own win32 WindowProc anyway, even if written in higher-level tech.

For example, if you want custom window controls, you need to use a WindowProc + WM_NCHITTEST to tell windows where the buttons are, so the OS can do things like display the window snapping controls when you hover over the "Maximize" button.

Sidenote: as a designer, its disappointing how many Windows apps are subtly broken in a bunch of these ways. Its not that hard. "Modern" UI frameworks generally don't do this work for you either, there's a real lack of attention to detail.