Problem is, you need to figure out a way to send and receive data from the backend (an API), a way to show validation errors, a way to do i18n and translations, a way to access your database (an ORM or similar... no, no raw sql, thanks , a way to do authentication and authorization, an admin or a cms to browse your database,a way to do background jobs or long running tasks, a way to send emails, a way to write CLI maintenance tools, a migrations system, and so many other things I could spend all day talking about.
Of course for a todo list list app or a landing page it is an easier solution, but real world applications need a lot more things you'll have to figure out by yourself. I'm telling you this from my own experience with Next. What we used to give for granted with Django suddenly we found out we had to rewrite a TON of code ourselves, and glue togheter many libraries of varying quality and maintenance, and the result wasn't much better or documented or performant or enjoyable as what django used to provide us with.