Every third-party auth provider is a dependency you don't control.
The modern authentication landscape asks you to choose between complexity and dependency. Build it yourself and it becomes a maintenance burden. Use a third-party provider and you've handed session management, credential storage, and availability to someone else's infrastructure.
Passwords introduce exposure. Social login introduces platform risk. Magic link providers introduce deliverability dependency. And when the auth layer goes down — everything behind it goes with it. None of this is acceptable for production software that runs real operations.