Build full fledged Web API [on hold]

I want to build a software as a service using Web API in VS 2015. I intend to handle virtually all aspects of the app like users' permissions, user roles, business logic, user navigation menus, user authentication in the Web API. With this I want my User interface designers to concentrate on their respective UIs(web, IPhones,Blackbery, android, windows form) and then call the web api for all CRUD operations. Data that will be moving from UIs and the API may be large and will include images. Is this approach the best. Please i need the pros and cons especially from those that have thought of or implemented my similar situation. Thanks