When comparing Vaadin vs Syncfusion JavaScript UI Controls, the Slant community recommends Syncfusion JavaScript UI Controls for most people. In the question“What are the best Javascript UI Widget Toolkits?” Syncfusion JavaScript UI Controls is ranked 5th while Vaadin is ranked 12th. The most important reason people chose Syncfusion JavaScript UI Controls is:
All components have been built as modules to enable selective referencing, so only the components and features you need are included in your application.
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Pros

Pro Compiles Java to JavaScript
Vaadin uses GWT to compile Java code into JavaScript. This means that developers using Vaadin can write both frontend and backend code in Java.
Pro Easy to use
Vaadin has a 'design mode' which allows developers to drag and drop components into a canvas and then provide the logic for every component. There are also a lot of third party tutorials and guides which help with the initial learning curve.
Pro Top notch documentation
The book of Vaadin is a must have for every developer that uses Vaadin to make web apps. It is an excellent reference manual and all around a great tool for every Vaadin related need. You can find and download the pdf online or get it for free in almost any Vaadin sponsored conference.
The online documentation is also very good. It has some tutorials and video guides.
There is also a vibrant community which sorround Vaadin, for any problem you may have there is a big chance that someone has already asked and answered that question on StackOverflow or in the Vaadin forums. If not, it will probably be answered quickly if you ask it.
Pro Modern, configurable theming
Vaadin comes with the Valo Theme, a Sass-based theme and engine that calculates styles based on configurable variables and making it easy to define a completely new theme with a few lines of SCSS.
Pro Easy to develop prototypes quickly
Vaadin has what is called the Vaadin Directory which is a repository of downloadable Vaadin components which can be easily used for development. To use something from the repository, you need to download the JAR file and add it to the project.
Pro Modular and written fully in TypeScript
All components have been built as modules to enable selective referencing, so only the components and features you need are included in your application.
Pro Responsive and Touch friendly
All controls are touch friendly and render adaptively based on the device they are on to provide optimal usage experience on phones, tablets and desktops.
Pro Built-in themes
All 4 major themes are available such as Google's Material, Bootstrap, High contrast and Office 365's Fabric themes.
Pro Fast release cycle
4 major releases per year and weekly patch release.
Pro Free community license
Cons
Con Weak scalability
Since Vaadin stores the UI state and logic in the server, this means that for every user interaction a request needs to be sent to the server and the client needs to wait so it can know how to react. This leads to higher traffic and load times.
