It should not make any difference if it has a flat roof, and is vertical or horizontal. Should not matter if it is an offset, like my Bandera vertical, with flat roof and heat rises directly from the offset bottom, like my large Spicewine, with flat roof and heat comes directly from the bottom with a large water pan deflector, and the uds, mine does not use water. Take something on the line of say a Good-One, which has the fire coming from along the back side, with a flat roof, and holds water, or my old Ok Joe offset with a barrel horizontal cook chamber. Heat and the gasses are going to rise to the roof, the only thing that could make a difference is an inefficient fire, and , or a lot of moisture from a water pan. Heck I have had it happen on my weber kettle, and my wsm.
One of the reasons I do not like using water, is it just really can make a big mess at clean up, plus when you get that nasty stuff on you, it is hard as hell to get off, don't even try getting it out of your cloths. I find it hard to remove from grates and the inner walls, it can concevable drip, if allowed to accumulate, but I admit I am a messy cook.
When the going gets tough, I take more fiber.