Its hard to say? We tried deep litter with our first coop and the previous homeowner of my current house was using deep litter in the goat house which was also shared with about 15 chickens. In both cases, the underlying litter would rot. Probably because of the high sustained humidity, combined with the intense heat that tends to stay above 90 from Memorial day to October. It didn't matter how good the ventilation was. I would even leave the main door to the coop completely open just to provide extra ventilation. The rotten poop & bedding would draw flies. Thousands of them. And in my first coop (but not the goat house), I even started to get moldy bedding that would get kicked up because of chickens digging in it, even if I put a thick 4 inch layer of new bedding on top. It only took me one season of that experience to say NOPE, NEVER AGAIN.I'm in the South and so far deep litter is working great! Maybe it's because I built a hoop coop and the structure is so open?
Now...here's one thing that may be a contributing factor. My first coop used plastic industrial pallets for the flooring. And the goat house is on subfloor laid over wooden pallets.
However, I kinda use deep litter in my duck coop which is also a hoop coop. In the summer, it is just a regular dirt floor/bare ground. No bedding at all. But in the winter, I do put down straw. Every week, I pile more straw on top. My hoop coop is vented along the top but also along one side where I have the tarp rolled up to provide extra ventilation during the summer since it gets so hot. I typically put it down for the winter.
Anyway, my point is that this method in my hoop coop doesn't stink, rot, or mold the way it did in the other 2 situations... But the differences -
1. It's on bare ground instead of flooring
2. I only do deep litter in the winter when it's not hot & humid.