> You're missing a major feature - cross platform. WPF is Windows only.
You’re missing the context. The OP has created that Electron-like thing to embed into a windows-only WPF app.
> the visual designer in Visual Studio was really buggy
Expression blend was, and still is, much better. It’s not especially friendly towards programmers, initially I had to read quite a lot of documentation and articles. But now I use it a lot (albeit not exclusively) and enjoy doing so.
> things like event handling and synchronization were fiddly to get right.
For me, MVVM works in 90%. The trick is to recognize the 10% of things for which it doesn’t, and do something else like code behind, or programmatic visual tree manipulation/generation, or something else.
> I think they're a great option for desktop UIs
If that desktop UI is a thin interface between user and a web server/cloud, maybe. If that desktop UI actually does something locally, I don’t think HTML is a great option: shell integration, WinAPI interop (both need COM interop). Also JavaScript isn’t exceptionally good general-purpose programming language especially for medium to large projects, e.g. MS had to create a TypeScript on top of JS for their VS Code.