We used keycloak at the Accelerator I worked for. I used it to provide auth and SAML SSO to enterprises while giving our devs an easy to integrate module. It worked great. Theming the login/signup/forgot password pages was simple and easy but would have been easier for the teams if it was React. Nevertheless we built some amazing B2B SaaS software using Keycloak as our identity provider.
Just because of consistency with our design components. You don’t need react for a login page. If your app is already using react and has a lot of custom components, it would have been easier for the dev teams vs having to learn Freemarker.
I would use it again for sure.