Risk adjusted scoring is currently absolutely not viable. You underestimate the messiness of the healthcare system by a huge margin. We don't even manage to record basic vital signs consistently, so believe me when I tell you that you can forget about any kind of nice statistical trick given the weakness of our data gathering processes. Plus it's not a matter of objectivity. Surgeons will subjectively assess that doing easy cases will be better. And in addition, they'll be correct. That's what really matters.