Atlanta is the world's busiest airport. Although they do very well with the volume, if a storm approaches it has a dramatic impact to flights and often leads to more then just delays, but a lot of cancellations.
Also, depending on the airline you are flying, sometimes you have to make terminal changes and if your inbound flight is delayed due to departure airport (not ATL), changing terminals can take a quite a bit of time, depending on the arrival/departure gates.
That said, I think ATL is a great airport, but there is a dose of reality in the mix.