Grandparent explicitly asked about "conventional server-rendered web applications (as opposed to API backends)". IMO Wicket does a really good job of doing everything in a clean, consistent way (on the server), so you don't actually have a problem of mixing e.g. server- and client-side form validation (and while a network roundtrip is always going to be slower than local validation, the AJAX integration is very good and makes it a lot lighter than a full page load).

I go back and forth on whether that's a good architecture in principle - a client-side UI calling an API has clear technical advantages, and it's much easier to build first-class applications that can run in JavaScript than it used to be. But certainly if you want that architecture then I think Wicket is still the best way of doing it - indeed I'd say it's the best UI framework I've used anywhere, the model/component split forces you to clearly decouple your data from your UI in a way that's not as thoroughly enforced in e.g. Qt.