The problem with CO2 is it blows away - so you put the thing out, then it re-lights, then you put it out again... etc. I had this enjoyable discovery using a 10Kg CO2 in anger on a burning bin full of oily rags, used 2kg on the first attempt, 3kg on the second and the remaining 5kg on the 3rd then dragged it outside still very much alight and emptied a kettle full of water onto it which did the trick.
As has been said, it doesn't take the heat out like AFFF or water.