The list of missing features [1] makes it clear that this is unusable for any realistic desktop app. No local file selection, no platform drag-and-drop… no CSS filters for UI effects, even. Nowhere near production ready.
Ultralight dev here-- most work has gone into porting Ultralight to various platforms (game consoles and ARM64) the past 2 years. The main focus is less to replace Electron and more to get modern HTML/JS/CSS running everywhere.
The desktop layer (AppCore) is indeed missing some support but I have some cycles allotted to finish them in the next dev branch (1.4).
Yes-- WebCore has an internal Accessibility API that I'm going to expose in 1.4 to be either handled by the embedder (so you can determine the best way to display / navigate that information) or handled by AppCore (we provide stock implementations of most of the platform API there).
[1] https://github.com/ultralight-ux/Ultralight/issues/178