On macOS it is still comes up short on a lot of the GUI functionality. Stuff like window management, decoration, focus, undo/redo, finder tree type views etc. What Apple have implemented seems okay to me for iPad, iOS etc, but is not up to the standards users' expect for non-trivial native desktop applications.
With enough determination most of the omissions on macOS can be worked around by breaking the SwiftUI encapsulation and delving into AppKit or jarringly inelegant solutions. All of which are somewhat liable to ended up binned when/if Apple pull their fingers out. I had higher expectations of Apple, and overall, it's easy to understand why it does not have a stellar reputation.
As to why it's a bit rubbish on macOS; SwiftUI is largely a mapping to UIKit, AppKit etc primitives. At a guess Apple wants to reduce that number and are seeking to minimise investment in moribund technology.