> Is WinUI3 a flash-in-the-pan system like their other UI attempts, or is it decent and stable?

If you stay in the happy path, it's decent, better than it used to be. Microsoft does seem committed to it, they're slowly converting Windows apps to WinUI 3.

That said, the team is clearly understaffed; there are long-standing unresolved bugs, just search for "memory leak" on their GitHub issue tracker. Also, native, non-.NET support is definitely an afterthought, it's barely documented and the tooling is super awkward. But at least, unlike WPF, it exists.

WinUI 3 was a step back from WPF in technology, because it panders to the virtually non-existent Microsoft .NET mobile app market.

To remain compatible with Android and iPhones they removed or simplified a bunch of features, ironically stripping out HDR support just when practically all phones got wide gamut and HDR, OLED screens, etc.

In the era when mobile phones are getting amazing, Microsoft is still racing towards the bottom along with every laptop maker other than Apple.