At my company we've been slowly rewriting our UIKit app in SwiftUI. We were due for a rewrite when SwiftUI dropped. I'd say we're about 75% done.
For those "stupidly complex UIs", SwiftUI just doesn't have enough hooks for customization. And the improvements are too slow. Using SwiftUI has been bitter-sweet. It feels like you just can't use it to craft a high quality app that doesn't look like a generic iOS app.