Don't expect much adoption love.
This is disappointing to me, even though I'm no longer at Microsoft, because it means that Windows is now the only major platform where there isn't an official modern platform-native UI toolkit that people are likely to use. Win32 isn't really a good choice for new applications because it uses an antiquated graphics implementation (GDI). Win32 also doesn't make it easy to tweak or extend the accessibility implementation of a standard control. All of this means that for developers of native applications, Windows makes it harder to do the right thing with regard to accessibility than, say, Mac or iOS. If WinUI 3 actually took off, it might fix this.
The team still has a lot of work to do; unpackaged applications still aren’t supported, for one. And even once it’s production-ready (next year?), I’m not sure it will be good enough to use instead of web UI.
I do quite like how easy it is to build UIs that work for both keyboard+mouse and touchscreens. But overall it’s basically WPF with a few improvements, and the worst parts of WPF (styling, verbosity) are still there.
WPF is supported in Expression Blend, the tool helps with both styling and verbosity. For optimal UX it needs a bit of support from within the application, to provide design-time data. Ain’t terribly hard to do, and helps a lot when working on WPF GUI apps.
WinUI is not currently supported in Blend.
The XAML is incompatible as the rendering engine doesn't expose the same feature set.
Well, our use case is not to rewrite everything yet one more time, a use case that apparently is hard to understand.
Designers aren't supported, yet another please get in touch and explain your business case kind of answer.
I found WinForms quite usable.
This is kinda amusing because WinForms is very popular among game developers for making tools and 3D editors :-P.
For .NET developers, WinUI still represents rewriting the UI and for stuff like Viewport3D or HLSL support, the advice is to learn C++, make use of SurfaceImageSource and rewrite the stuff directly in DirectX.
Quite a change for LOB developers, then there are the missing components as well.
For C++ devs, the aging MFC is more productive than dealing with IDL files without any kind of VS tooling support, all components are based on COM (hello boilerplate) and requires manually merging generated files into the project.
They say they are listening to us, however they still fail to deliver what we complain about.
Meanwhile those that can afford it, will just adopt Qt, Delphi, C++ Builder instead.
So expect most people to still ignore these efforts.
Windows basically has lost a decade chasing a pointless unification of desktop and mobile platforms. UWP turned out a turd, no matter how you look at it.
Except they seemed not to have gotten the message and still are asking for rewrites, with less capable tooling.
EDIT: The only thing Reunion seems to be good for is as migration path into Win32 for UWP apps.