So what is the risk calculation based on? Presumably the doctors refusing to do procedures they deem high-risk are not determining completely at random. The risk calculation doesn't have to be perfectly accurate, and it seems to me as though heart procedures are better defined than software projects.

(And besides that, although software estimation is notoriously inaccurate, it is still done and written into contracts all the time, because it serves a useful purpose that outweighs its inaccuracy)