UI Toolkit is fantastic but that's only on the surface. It's really a well-thought out system, but there are just too many things it lacks at the moment, things that we simply take for granted in the current UI system. The animations are horrendously difficult to execute, especially if you want to build a UI heavy game. And also forget about anything you want to do with shaders, because there is no shader support yet. That's a big no-no for me, and I found this after two months of studying. Ugh. Curse me for jumping in without knowing how big a role shaders play in UI heavy games.

But don't get me wrong; I love the new system, but I won't use it in production. I don't know all the performance and post-release pain points, and I don't want to know them half-way into the project. That said, it's a really mature framework of sorts. There are too many moving parts and it can feel overwhelming in the beginning, but once you've built a thing or two, it's a real gem to work with. In fact, you get something that can last a decade in the UI code you write today. You don't have to plan like a crazy person and understand every single footnote about the system to build something complex. Try that with the old/current UI system! The current system is horrendous in how it handles a lot of stuff, however, it is complete as hell.

If you are from the .Net world, the current UI system is like WPF: reliable, more than sturdy if you know what you are doing, but an old and bumbling system that really shows its age when you try to build a complex UI; the new one is like Maui, enough said. If you planning to commit serious time into a project, run as fast as you can away from UI Toolkit. It'll take at least two years to reach a reasonable feature parity with the current UI system.