Every surface has it's own pluses and minuses for any group of dogs (by size, speed, etc). There have been some studies. Honestly I would love to see more and more in depth studies looking into benefits and risks for various groups.
Any time you run on grass, turf, dirt, or any mixture there of ruts and wetness can be a concern. In dirt/mixtures it's not only ruts/wetness but overall depth of dirt as well as dirt vs sand ratios that can impact some dogs more heavily then others. I will say depth needs to REALLY be looked at both at local and national events. It can quickly become not just unfair but a health/injury concern for all dogs but especially for our shorter legged partners...speaking as the owner of a dog who came back from a Champs with some significant overuse and strain issues from running in too deep of a dirt/sand mix. I've learned to become a surface snob and yep there are events we just don't go to because that is my choice).
Mats and astroturf...not all are created equal. There is a huge difference in the amount of grip vs slip between manufacturers (and it changes with age and maintenance). Though I will say I think overall mats and astroturf provide a more "level" field as far as footing challenges.
All have assorted risk factors for injuries, wipe out, etc... There is no perfect surface for every dog. We all have to pick and choose which surfaces fit our team best and weigh the pros and cons. I understand why champs tends to be on dirt...it's hard to find facilities with sufficient ring, crating, vendor, and camping space for a National event. I do think it would be nice to have it on a different surface now and then so teams who don't run on dirt or don't run well on it (despite practice) perhaps get a "chance" as well. But I understand the difficulties in finding suitable facilities.