I've found a few things in the process: 1.) Neither RN nor Flutter seems to be able to create truly native applications on iOS. I've never once seen an application made in either of them and thought it was a native iOS application. 2.) Unless your application must support both platforms, android (in an economic sense) is dead weight. I was shocked to see how bafflingly little android users contributed to our revenue. I've heard this from people in other companies as well. 3.) SwiftUI (and I assume UIKit) makes it really simple to create apps according to HiG. You can feel yourself fighting the framework whenever you deviate from what Apple wants you to do. I actually think this is a good thing.
I think Apple is doing something really smart here. They're not making SwiftUI cross platform, they're making it possible for you to re-use your business logic from your SwiftUI apps in Android apps.
The way see I see it they're saying — if you want to spend the least amount of time possible building a cross platform app use Flutter or RN. If you want to create a truly native experience on both platforms, but still re-use your core business logic, Swift is your friend.