We are currently re-writing the UI of our flash card vocabulary learning app Wokabulary with SwiftUI.
Wokabulary is available for iPhone, iPad and Mac; so having a platform independent UI system saves us a lot of time.
However, we face quite a few challenges with SwiftUI especially on the Mac. For example performance of SwiftUI on the Mac is often worse than with AppKit (e.g. lists). Also the documentation for Mac specific features is very sparse and some things just cannot be done (e.g. customizable toolbars).

So we are wondering: Are you using SwiftUI for your Mac apps in production? What Mac apps have you built or are you building in SwiftUI? What challenges do you face?