I built a similar "login as a service" three years ago, and the biggest I came across was that any modern web framework already includes authentication and user

I built a similar "login as a service" three years ago, and the biggest I came across was that any modern web framework already includes authentication and user management out of the box. And user management that is guaranteed to integrate with the rest of the framework. It was a really difficult sell trying to convince developers to use my service rather than their own framework.