The advantage is that you keep all the smoke, mess, and fire danger outside and away from the house.
The down side is the cost. A factory made one costs several thousand dollars. We burn wood to save money, thus the spending of several thousand doesn't support that goal.
I will show you how I built mine for almost no cost.
I ran the first one of these all last winter with no problems at all. I didnt buy a single whiff of gas for the whole winter. now I am making this one for a friend
Step 1: The fire box
* First we start with removing the outer cover, insulation, burner assembly from the water heater. all thats left now is the tank.
*Next, I cut out the flue running up the center, and all the pipe fittings, and welded patches over the holes.
* Then, I reused the flue from the center and welded it to the outside of the tank. I welded a plate on the end for pressure testing.
* Now we have to pressure test the tank for leaks. For this, I welded a small pipe fitting in a convenient spot and installed an air valve. this is like a tire valve stem with pipe theads on it. they are used in campers to pressurize the water tank. we only need about 5 psi. with the tank pressurized, I used a thick solution of dishsoap and water in a spray bottle to find the leaks.