Yes, Flutter is useful for when you have GUI heavy apps, not necessarily ones with platform integration like menu bars and cameras, although that is progressing pretty well I'd say, people are writing plugins for platform capabilities.

Regarding Windows and Linux, I too was surprised at how well everything worked. You can't build to production yet for these two platforms (officially at least, see flutter-rs for unofficial implementations), but the debug builds still work as well as the Android and iOS emulators. I used them on a lower powered machine to run my Flutter apps and it was great that they worked well.