Too much magic. Debugging this would be a real pain with everything happening over websockets
You can debug all your logic on the server.
I'd guess a/the big drawback would be no offline capability. But if you're fine with that (many are), then why not?
You are correct that, since kwebsites are server-driven, there is no significant offline capability.
But then the requirements of many websites often rule that out anyway, my strong suspicion is that this won't be a dealbreaker for many people.