This is a relatively compact plugin that provides custom pages for handling user authentication and registration. Although it therefore works with security-related functionality, it does use the provided WordPress functions for login and user registration requests, so the key implementation points should be safely handled by WordPress.
Reason for the 'Use with caution' result
The plugin has been given this recommendation at the tester's discretion:
It does give detailed error messages, such as whether an email exists for a registration attempt. This may be helpful to users but also gives information that may be helpful to potential attackers.