This could be a phone call between Hearne & me.
There are all of the above reasons for sure. And there are the researcher research issues, like removing all of the variables for any given study. I'm going to hunt down the data on S&C injuries or deaths. An issue there is how do you compare that to something that did not happen, i.e: fewer deaths during free flow clearing?
There are studies being done but sometimes they're almost neutered. College students don't always make good badguys for shooting purposes. In a recent one I looked at, 230+ initial participants but only 170+ usable runs - @ 60 of the student participants didn't shoot. Generally, I like the research coming out of ALERRT, their book on evaluating room entry tactics for not-SWAT is worth it just for their methodologies. One of many things in it is their addressing the +/-20% hit rate claim.
Back on-topic ... there's been some info shared with peer cops purportedly about things that were seen, learned. If the stuff is factual, it'll be interesting to see it get released. Sadly, given the drama MPD has suffered of late, I don't know if they'll be able to get that information out and have it gather traction.