We’ve recently launched a pretty decent sized app using Razor Pages for the front end and MVC controllers for the API for client side components. My experience has been this:
The pages paradigm works well when your content is structured around the idea of actual “pages” on the site. Think about things such as a Contact Us or an About or even a Login page. Sure, those could be done via MVC, but MVC is really unnecessary. A simple page will suffice. Leave the controllers to more controller’ish things like a product catalog or a user database.
If your MVC architecture revolves heavily around your view structure, razor pages is probably a good fit. You can still use the MVC bits for API related stuff, but the benefit of pages is that your front end structure becomes more explicit and less implicit (“convention-based”) like with MVC where each action could or could not have a view that is typically named after the action.