I went with IdentityServer4 on a recent project over Keycloak and Gluu[1] for that reason and because it was in the same stack as the rest of our ecosystem.
IdentityServer certainly has better documentation for its extension points at the moment, but the tradeoff is you have to build everything yourself. Keycloak comes with a built in admin UI, account management UI, TOTP and WebAuthN support, the list goes on. You have to go out of your way to build these or search for a mismash of plugins for IdenityServer to get everything Keycloak provides out of the box.
And while the extension points of Keycloak aren't super well documented, literally all of them are in a dedicated Maven module [0] making it easy to just browse the code.
