I am very surprised by this. I have successfully and interchangeably connected to many OpenId Connect providers, all without any issues. Providers like Azure, Ping, Octa, Auth0, ….
No - I am not an employee of Zitadel. That just the OpenID Connect Solution I decided to settle with. Keycloak and Ory Hydra looked to heavyweight for me.
Agree, OIDC is the simplest and most versatile way to logon users and can be used in other scenarios. The discussion usually is where to store the tokens.