We built our startups web UI on GWT. It was super easy to get started and we had a decent looking UI in no time. Then newer JS frameworks came along and our site looked obsolete and crappy by comparison (I'm talking circa 2014-15 or so). We started looking at making the GWT code look good visually.

The pain was too big, styling this thing vs. just using ready made JS and HTML to create a simple UI. We eventually dumped GWT which was easier than going through the pain of adapting the UI. It seemed the community was dead and Google abandoned this long ago. Vaadin seemed nice but overly focused on the server aspect which we didn't feel we needed. Even they moved off of GWT eventually.

Today we still use TeaVM as part of Codename One itself to create web UIs. It works great and since we have the UI aspect working its good. But this works more like a Flash applet and isn't meant to be a website or typical web app.