It's convenient, until you've written 90% of your app in record time and then end up so blocked on the remaining 10% you wish you would have just used UIKit.

I love how quick and easy SwiftUI is for simple, Apple-like / stock app interfaces. But the second you try to do something outside of their guard rails (some of which you don't even see until you've gone outside them), SwiftUI quickly makes you realize you are swimming against the current. I also find debugging SwiftUI to be exceptionally painful compared to UIKit.

As others have said though, this improves with every release so I'm confident things will only get better.