Agreed. It’s difficult enough with ordinary cars – driving style, journey types, road conditions etc. I’d say with a PHEV adding in the battery range factor, it would be impossible to produce anything meaningful.
I do a regular airport run in my current X1 and note the mpg over the return trip The route is optimum driving for fuel economy. I will be going again in a few weeks but in the new XC40, so I will see how it compares.