I’ve been been using UI toolkit for a couple months now and I can say I’m having a blast. The API is powerful, and I’ve found a way to do everything I need so far. And I’m doing a fairly complex UI (a full windows OS shell).
That said, I think you should consider if the UI you want to do is structurally complex or visually complex.
If it’s the first, I’d say go for UI toolkit. It manages complexity like a champion.
Now, if it’s the second, UI toolkit is not there yet for the visual stuff. If you’re gonna need non-rectangular shapes for your elements, particle effects, complex animation, full support for TextMeshPro, or custom shaders, I’d stick with UGUI.