Yes, it's highly route, day, and time dependent. I often have a difficult time trying to SDC when traveling for work because these business-heavy flights are frequently sell last minute tickets and are full, and so Delta heavily restricts fare inventory available. On the other hand, when traveling for leisure, I'm frequently able to SDC because I seek out travel on days/times that are cheaper. So, ironically, it's often most difficult to SDC on tickets with relatively higher fare classes.