It’s missing a lot of features that are currently in UIKit and the way that views are structs and it’s a declarative UI means it can be hard to add custom UI and work around missing features. You have to wait for Apple’s developers to implement things. In UIKit you can just start overriding things (even if you probably shouldn’t).
Also in my experience performance can be awful in ways you can’t work around or fix easily, if at all. e.g. The framework is supposed to diff and determine if things have changed, if two views are equivalent, etc. This can slower than just forcing the system to consider everything changed and nothing is equivalent so it should reload everything. This might be a bug, but sometimes this diffing can happen even when you change nothing. Ive gotten incredibly stuttery performance when scrolling. When I hook it up to the performance analyzer I see it’s trying to diff the whole screen multiple times every frame for static, scrolling content. But this only happens with finger scrolling. Programmatic scrolling (e.g. with the crown in the watch) doesn’t do this and is stutter free.
Finally, Xcode support is bad. It still gives inscrutable errors (that are usually just a misplaced parenthesis or brace) and the type inference system can peg up the CPU at 100% CPU (and then the system will throw up its hands that the type is too complicated to figure out.)
Prototyping in SwiftUI is so, so much faster than in UIKit though.