Vaadin is open-source, free-of-cost, and used by at least 150,000 active developers. Many components are bundled, and many more components can be plugged in via the emerging Web Components technologies. Vaadin 6, 7, and 8 were based on GWT, while Vaadin 10 and later (Vaadin Flow) has been re-engineered to be based on Web Components.
Inherently more secure as no business logic is running on the client-side. The state and business logic of your app lives only on the server-side.
Powerful tooling and a rich ecosystem comes with the Java world. This includes potent IDEs, wide-ranging unit testing with JUnit 5 and its many pluggable testing frameworks, testing tools available (for commercial fee) to support UI testing both logically as well as visually, and participating in continuous integration workflows.
Vaadin scales very well because it leverages the speed, reliability, and superb threading of the modern Java Virtual Machines. Caveat: Because the state of the app for all your users lives on the server-side, you do need a substantial amount of RAM and CPU on the server. How much depends entirely on your particular app and number of simultaneous users.