There's 2 ways to grill using a chimny, over and under. Over, you run the risk of grease dripping and flare-ups. Under, make sure that you give the chimney a good shake to knock the ash off, and the food has to be contained within the rim.
I make steaks sous vide, and have balanced a grill grate over the top to sear them off (half a chimney full.) At nightfall, the 1/4" stainless rod is just starting to glow in the center. My IR thermometer goes off scale at the top rim, and an inch or so below, is at almost 900 degrees.
Downside is that you're fairly limited on how much you can grill at once. I use the biggest Weber chimney and it's just barely big enough to do a couple of small steaks at the same time. A big ribeye? Forget it.