ORY Hydra tries to solve all of OAuth 2.0 and OpenID Connect uses. There are, however, some limitations.
ORY Hydra has issues with MySQL <= 5.6 (but not MySQL 5.7+) and certain MariaDB versions. Read more about this here. Our recommendation is to use MySQL 5.7+ or PostgreSQL.
OAuth 2.0 Client Secrets are hashed using BCrypt. BCrypt has, by design, an maximum password length. The Golang BCrypt library has a maximum password length of 73 bytes. Any password longer will be "truncated":
For more information on this topic we recommend reading:
ORY Hydra does not and will not implement the Resource Owner Password Credentials Grant Type. Read on for context.
This grant type allows OAuth 2.0 Clients to exchange user credentials (username, password) for an access token.
You might think that this is the perfect grant type for your first-party application. This grant type is most commonly used in mobile authentication for first-party apps. If you plan on doing this, stop right now and read this blog article.
The ROCP grant type is discouraged by developers, professionals, and the IETF itself. It was originally added because big legacy corporations (not dropping any names, but they are part of the IETF consortium) did not want to migrate their authentication infrastructure to the modern web but instead do what they've been doing all along "but OAuth 2.0" and for systems that want to upgrade from OAuth (1.0) to OAuth 2.0.
There are a ton of good reasons why this is a bad flow, they are summarized in this excellent blog article as well.
Auth0, Okta, Stormpath started early with OAuth 2.0 SaaS and adopted the ROPC grant too. They since deprecated these old flows but still have them active as existing apps rely on them.