Po-tay-toe Pah-tah-toh
I would say that if you used a WSM, Cajun Bandit, or even a ElCheapoBrinkmann without the waterpan, you would have the effects as a UDS.
Not that this is the definitive answer to your question-but read the description from the Big Drum Smoker homepage.
http://www.bigdrumsmokers.com/
Personally, I love the drum and the flavors I get from the meats I cook on them. I've got four of them, 15, 55 x 2 , and an 85 gallon UDS's. Also have a 22" , three 18" WSM's and a Cajun Bandit kettle conversion.
I've used the big bullet and Cajun Bandit direct without the waterpan to cook chicken on the grate and with Rib-O-Lators and rotisserie baskets as well as on the UDS and felt the results were comparable, but for some reason enjoyed the drum cooked more.
Some people don't like the drum taste and add a waterpan or claypot base and make their drums an indirect cooker like a WSM and are fine with it-I know JD doesn't care for the taste, and I have eaten Chicken and Ribs he has cooked on on his drum that are every bit as good as what he cooks on his bullets!
We basically stopped using the drums in competition, because we started scoring higher cooking indirect. Negative comment cards from some judges kind of sealed their fate-and a few seasoned comp cooks around here asked me why we stopped using them and I told them why, and they told me that was the same reason they went back to using the pans in their WSM's!
Go figure?