I am from OK and have bed on both concrete and dirt/clay floors. I see the benefits of both. The problem with the dirt/ clay is the horses are able to create craters in their stalls, whether that is from peeing or from digging. Even with rubber mats, you still end up with squishing of pee under them if you have a heavy peeing horse, esp since clay isn't the most draining. If I ever have to bring in more clay to fill in stall craters again it will be too soon.
Currently the barn we are using has concrete floors with drains (though the drains I am sure need cleaned out after many yrs of use) along with rubber mats on top. We bed on paper shavings and will bed anywhere from 5 inches thick down to just enough to soak the pee. Stalls are much easier to clean, and we have not had any soundness issues.