I've been commuting from ATL-ORD for the past 3 months and have had zero problems. I've somehow made it on every flight I have attempted. I did have to do MDW once due to ORD weather cancellations (high winds). I also only have UA benefits. There are over 25 fligts a day on 5 different carriers (American, Delta, Frontier, Spirit, and United), so plenty of options . ATL-IAD is much tougher. That commute usually has about 10 overbooked flights a day between Delta, Frontier, and United. There are also a TON of commuters on ATL-IAD.