It is fine for small toy apps (think the calculator), but it is missing a lot of functionality to build a full fledged app. Think collections, which is the backbone of most apps.
UIKit is very mature and has evolved to a really good UI platform. SwiftUI is not even close to unseat it. Maybe in few years it will mature as well, but for now UIKit is the way to go.
I only ever played around with making a toy app while following along with Stanford's Swift UI course[1], and even just the basic things I was doing there, I ran into multiple frustrating/confusing bugs.
As if it wasn't difficult enough to wrap your head around learning it in the first place, running into bugs makes it even harder because you think you're doing something wrong until you finally ask online and get told by people more knowledgeable than you, "I don't know why that's happening, wrap your view in a container and it'll work"
At Beam[0] we are building a web browser that uses SwiftUI almost everywhere except for the text editor.
So far its been great to build UI and animations with. Each release gives SwiftUI more apis and features, so there are less and less older style apis to wrap in SwiftUI ourselves.
UIKit is very mature and has evolved to a really good UI platform. SwiftUI is not even close to unseat it. Maybe in few years it will mature as well, but for now UIKit is the way to go.