When gauging airport reliability it's important to separate out delays propagated from other airports in the system from delays caused by weather, capacity, or operations at the airport in question. For example, I've experienced plenty of delays at IAD, but I agree with the label of "Generally reliable" for it because the delays usually originate somewhere else. Either it's inbound aircraft delayed because of weather or operations at its origin, or it's an aircraft sitting on the ground at IAD because there's a ground stop or flow control at the destination-- particularly when that destination is EWR, JFK, or LGA, which are all plagued by both weather delays and overcrowding at peak times of the day.