I agree with you. I looked into Persona to implement an SSO for our products (provider and client). I've never been able to understand OAuth in the context of an authentication mechanism so I left it.

However, Persona has very poor library support, especially for providers. The support channels are also very small, so you're not likely to find other people fixing the same issues.

I ended up just hacking an OpenID provider libary to get the result I wanted. It's a real shame because Persona seemed to be designed for circumstances very similar to what I wanted to do.