It's really hard to use with Win32 / the traditional Windows API too. Often APIs aren't exposed or documented -- consider the APIs to turn on the Acrylic or Mica looks in a window background, for example. There are a few Github repos that show how to do it but using reverse engineered APIs or constants.
By comparison when Windows 7 was released, enabling glass was documented and usable from any language or framework.
It's also not currently possible as far as I know to mix new controls and old controls in one window very easily - there are islands, but they seem on the macro scale (from when I last looked.) This makes updating apps difficult: changing a UI is an all or nothing upgrade per window. You can't just easily add WinUI to an existing WinAPI window in an app using a non-C#/Microsoft language. So people who have existing apps or use a non-MS dev environment are faced with enormous barriers.
The ties to Microsoft languages and IDEs are also troubling. It's not technically MS-only, but it is _effectively_ MS-only.
This is my personal account (I don't have a professional HN account) but I work at a company that produces a dev environment and IDE, and we run into these issues. I find that searching for my HN username and LinkedIn ("vintagedave LinkedIn") will likely let any reader (the OP, any reader, or u/
fassssst if you'd like to get in touch?) find me, and I'd be happy to speak on a personal or potentially professional level about issues with WinUI and what could be done to make it more accessible across dev environments and more easy to convert to or upgrade to.