I think a good benchmark for most APIs is if someone, who isn't a subject matter expert, can come in and understand the facade the system exposes. For most LDAP/SSO/IdP/etc systems I'm completely and utterly lost without spending hours reading manuals and even then it's iffy. With this project I looked at this graph and instantly knew their head is in the right space:
https://kanidm.github.io/kanidm/#why-do-i-want-kanidmIt seems like they are reinventing the API for what identity management should be and then calling that API from points of enforcement (LDAP, RADIOS, etc). I have a feeling, as long as deploying this is simple, it would be amazing especially if their API exposes a limited amount of data that could be stored. It seems like from this graph that "Account Data References [kanidm]" which is a great sign: this is not a shared database, this isn't a complicated spec, this is a service that manages a mapping of login credentials => (UUID, metadata) in a secure way and integrates into "everything". You can then run whatever things on top of that which you need (email, corporate profile, etc) which, if this API is simple enough, isn't too hard to build or buy especially if the SSO token contains a display name, username, and email since, in my experience, that's all most websites ever use from a jwt since the entire space is too unstandardized on everything else.