My personal hunting with clients for buffalo is somewhere under fifty, I'm not a high numbers guys where buffalo are concerned. I've shot quite a few myself and I've been part of a PH "team" where hunters have taken Buffalo and other game. My opinion is not based on the kind of resolution that you can take to the bank like some of my other opinions on bullets, rifles and game. For some 10-20 events are a good tally to base facts on. I prefer more then that, much more. Since the loss of Zim for my hunting business I've not hunted Buffalo nearly as much as before. It's a shame that Mugabe has done this, but I won't take my hunters there any longer. In RSA we can hunt Buffalo with no problem, just not as many hunters for them.
However having said that, in all the buffalo, I've personally seen shot, and shot myself only a single one has dropped at the shot. It was a spine hit bull. I've seen at least a dozen shot through the heart and have posted the photos here of me holding the heart of a cape buffalo in my hands with a 375HH bullet through the center. None have dropped and most have gone 100 yards or more, many much more! It would be a freak situation to have only heart lungs hit and drop a bull in it's tracks. My guess is that a fragment of the bullet or some bone cut the spine. Otherwise, I see no real likely hood of a bull falling in it's tracks from a shot placed in the heart lungs.
Stranger things have happened though!