To me, the biggest problem with javascript as a backend language is - especially in large teams: people tend to mix backend code with frontend code after a while.

Looking at next.js for example. There are so many ways to fetch data for a page: getStaticProps, getInitialProps, getServerSideProps and the API handler (I hope I didn't miss any now). Each single one of those uses their own naming concepts (context vs req/res, query vs params). It is very easy - especially for newcomers to the frontend - to break things or leak data by not following the concepts of the frontend.

By dividing backend and frontend on a language level, you get a clear boundary on an API level which helps developers having a clear focus on their actual problem scope.

I guess you could solve the problem by having two node.js applications: one providing API endpoints and one taking care of the view. By then, I'd just switch to a properly type-safe language for the backend.