SwiftUI is still a little rough in some places though it is growing fairly quickly. If you are doing simple UIs where there are already UI components built, it works well, but if you need more custom or more complex you get into the weeds pretty quickly.

Catalyst is helpful for the near term (next few years?) to bring over more complex iOS apps that use the iOS UI framework. That gives developers a product now while they work on a SwiftUI version or whatever direction they end up going.