Comments: My overall experience with Vaadin is good. I have completed three small projects with it. And now I have one quite big project. I will use Vaadin with my future projects.
Cons: Of course, there are few downsides. Firstly, I didn't like the migration from Vaadin 8 to Vaadin 14. There were a lot of improvements. Quality of components went down for a couple of months. It seemed, that almost all Vaadin code was rewritten. I needed to learn almost everything from scratch (many components were the same, code for components stayed almost the same). Also, I wish that there were more books for Vaadin.