The down side is that you don't own any of the users. You don't even know who they are. Is user management for an API / backend only service that difficult th

The down side is that you don't own any of the users. You don't even know who they are. Is user management for an API / backend only service that difficult that it's worth giving over all your users to a 3rd party service which simply forwards on their requests? Throw up a POST /user endpoint which returns an API key, and implement auth and... you're done. I see value in the payments portion.