The main difference of using GWT or J2CL transpiler and JSF or Vaadin;

With GWT / J2CL you transpile your Java code into JavaScript and your web app runs on the client-side, directly on the web browser. JSF / Vaadin is server-side model, so your web app runs on the “backend”.

Advantages of client-side model:

• Scalability: put your presentation processing on the client, so the server only processes the business logic.
• Best API architecture: if you are doing REST-based architecture you can consume the same APIs from your UI.
• All those #PWA (Progressive Web Apps) advantages

Hope this helps…