I am also having issues with this feature.  What we need are two features with AI / BI dashboards.  
My web application will manage the user sign in.  I want to have a service principal that is authorized to run the AI / BI dashboard. Then I want to pass in Filters from my Web application that will pre-filter the data for the User to see in the Dashboard.  

I'd be ok If I could do this using a service principal or a Service account user something like (embedding@email.com) then using that "service" account user to embed the dashboard every time.  

I would expect my app to handle the user sign in, and figuring out which filters to pass into the embedded iFrame.