A crucial downside compared to wx is that U++ doesn't use native widgets, at least on Windows. And unlike Qt, it doesn't implement platform accessibility APIs. This is a show-stopper for blind users, because their screen readers can't identify the types of the widgets, tell which one has keyboard focus, etc.
It says "cross-platform" but every screenshot is Windows, the only installation instructions are for Windows, and there's no mention of any other platform except Android, which is not yet supported.
the only installation instructions are for Windows, and there's no mention of any other platform except Android
You could also edit your comment to remove these two completely false statements... The Linux instructions are very easy to find any I don't even know how you managed to miss them!