Bringing in a library is different from building it from scratch. Pretty much every language has solid OIDC frameworks now.
OIDC is great for greenfield and much better than writing user signin flow every time.
OIDC is great for greenfield and much better than writing user signin flow every time.
That being said, if you're working on a greenfield app where you need auth, using an OAuth/OIDC server is much better and faster than creating yet another membership system for that app. That's a major advantage of identity federation.