I've been contracting on a "modern" mac app in SwiftUI for the last few months, one of the issues I see is that you can't actually get that far without AppKit right now unless you have the luxury of supporting only the latest macOS. Even then people often just don't upgrade their OS until they buy a new computer so you're stuck with whatever version of SwiftUI. It's a bit of a weird time for macOS development.
AppKit may go away eventually, but _today_ it's still required to write a good Mac app even if you can use SwiftUI for large portions.
My experience on this project has also taught me that AppKit can be made to play quite nicely with SwiftUI. The AppKit code is encapsulated well, and from the outside gets instantiated and included exactly as if it were a SwiftUI component.
I’ve bridged several UIKit components into SwiftUI and it’s been fine. You have to be careful with Auto Layout, my approach was to divide into separate UIView and then wrap those in UIViewRepresentable, thus leaving the layout to SwiftUI. But that was the main thing to keep in mind.