As a so-called "full stack" developer, every project I've worked on in which the frontend and backend developers were clearly delineated has led to a mismatch in needs between the two.
The frontend devs asking for the API they need for the UI they want to achieve, and the backend devs having to predict the ways the frontend will need this data for now and in the future as the UI evolves and as the backend system is built out.
The frontend devs are at the mercy of potential backend choices affecting the "feel" of the UI over time, and the backend devs at the mercy of their data model, infrastructure, and system design choices not meshing with the future needs of the frontend over time.
I know the projects this sort of thing has gone bad for me on had other communication issues, but I still don't know if I could deliver a solid end product nearly as effectively without being able to jump into either end, understand the problems and desired outcomes, and be able to help both evolve in a maintainable and scalable way, together.