In my view the user-specific parts and frequently updating data should be rendered on the client side. The pre-rendered HTML should contain the less-frequently changing information that is important for SEO.

Given the option, and pretending it were just as easy to implement either, why would you wait until the document loads to fetch resources via Javascript when you can render the content on the server?

The article points out one downside: React's render-to-string faculties are slow.