Be it Keycloak, Gluu, FusionAuth - I’m in total agreement to pull user management out of your app - even if you are a keeping things as simple as a rails monolith.
The cognitive overload of having to align PM’s, ops, internal support, etc with a custom solution is worth any of the downsides.
If you’d sat through any sprint planning or architecture exercise in a sizable org for auth, these can be a magic wand.