Zitadel is switching its license from Apache to AGPL3, that’s at least one difference. The license change caught my attention because I also use Zitadel, this project is MIT.

[0] https://zitadel.com/blog/zitadel-v3-announcement

I think (not sure though) there is another difference to add here. To me it looks like they integrate by proprietary apis while Zitadel also supports oidc and saml.

But I have not checked their docs, so I could be wrong.