Swing is the perfect example that such approach simply doesn't work.

At the bare minimum, you need to use the native widgets on each OS.

It's baffling to me that Flutter didn't even learn this lesson from 20 years of agony with Swing.

Most interesting apps tend to use custom Android views, layouts and behaviors to create a unique, but consistent look and feel.

Flutter can do the same. There is not much difference, except that Flutter strives to do the unique part better.

On the Windows desktop your application must be coherent with other applications, since they share a big screen, or rather a workspace.

Either way you want to adhere to the design language of the OS, because it tends to work better than what you can come up with, and also provides familiarity. On Android you don't have to use the prebuilt widgets to adhere to the material design.

Flutter also comes with widgets that look and feel very similar to the Android and iOS ones, so you don't have to reinvent the whole wheel.

When I first got started with Java at a time when Javalobby was my Hacker News, there were endless rants and debates about Swing and native widgets. Yet, 20 years later Electron seems to have won the cross-platform platform and no one seems to care about native widgets - only memory usage!

Swing is the perfect example of devs that don't bother to read documentation and just go with defaults.

It can be made to look quite well, but yes the defaults are not the best and it requires graphics programming knowledge.

Qt is exactly the same, it does not use native widgets bit reimplements them (and leverages the system APIs to get correct colors, icons, and sizes). but everyone seems to be hell-bent on reinventing qt every three years so ... :p