1. Authentication and Authorization: Devise is a comprehensive authentication solution for Ruby on Rails, providing a set of easy-to-use authentication features. It handles user registration, session management, password encryption, and more. On the other hand, ORY Hydra is an open-source OAuth2 and OpenID Connect (OIDC) server that provides authentication and authorization services for microservices and web applications. It supports various authentication methods, including OAuth2, OAuth2 Client Credentials, and OpenID Connect.

2. Scalability: Devise is designed primarily for single-server applications and is not inherently scalable. It relies on session-based authentication, which can become a bottleneck when dealing with a large number of concurrent users. ORY Hydra, on the other hand, is built with scalability in mind. It uses stateless JSON Web Tokens (JWT) and leverages a distributed architecture, making it highly scalable and suitable for handling a large number of authentication requests across multiple servers.

3. Customization and Flexibility: Devise provides a rich set of default features and configurations, making it easy to get started with authentication. However, customizing and extending Devise can be complex, as it uses a monolithic approach where all the authentication logic is handled by Devise itself. ORY Hydra, on the other hand, offers much more flexibility and customization options. It provides a modular architecture and allows developers to choose the components they need, making it easier to integrate with existing systems and tailor the authentication flow to specific requirements.

4. Support for OAuth2 and OpenID Connect: While Devise focuses primarily on traditional session-based authentication, ORY Hydra is specifically designed to support modern authentication protocols like OAuth2 and OpenID Connect. It provides all the necessary endpoints, workflows, and functionalities required for implementing OAuth2 and OpenID Connect flows, including authentication, authorization, token issuance, and token validation.

5. Integration with Microservices: Devise is a monolithic authentication solution that is tightly coupled with the Rails application it is integrated with. It may not be the ideal choice for applications built using a microservices architecture, where authentication is often decentralized and handled by separate authentication services. ORY Hydra, on the other hand, is specifically designed for microservices and can handle authentication and authorization centrally, making it easier to integrate with other services and share authentication across multiple applications or APIs.

6. Security Features: Devise provides a range of security features like password encryption, account lockouts, and CSRF protection. However, ORY Hydra places a stronger emphasis on security and provides additional features like client authentication, user consent management, token revocation, and token introspection. It also supports more advanced security mechanisms like two-factor authentication (2FA) and multi-factor authentication (MFA), making it a better choice for applications that require higher levels of security.

In Summary, Devise and ORY Hydra differ in their scope and focus, with Devise being a comprehensive authentication solution for Ruby on Rails applications and ORY Hydra being an OAuth2 and OpenID Connect server designed for scalability, flexibility, and integration with microservices. ORY Hydra provides support for modern authentication protocols, offers more customization options, and emphasizes security features, making it a better choice for applications with specific requirements in those areas.