> Having the server return plain JSON means the APIs can be reused across products effortlessly and also means that all style changes can be done in the same codebase.

How many products actually share the same server backend? Do they all organise the same data on the same pages? If no, then you already need per-product APIs to avoid making O(N) fetches from the client side Having your backend be aware of what is being presented is rarely a bad thing