I'd think it's probably cord length. I am lucky in that I have an outlet about halfway down my pool up on the deck. So as long as the robot's cord enters the pool somewhere sort of close to the middle of the pool I would think it would be fine. that would allow it to go 30' in either direction and across the pool and the cord length shouldn't be an issue.
Keep in mind that the robot's patterns are generally random or driven by some set of gear inside the robot so it would go X feet, maybe turn one way or the other, go Y feet, repeat in seemingly random ways until the cleaning timer is done. My pool is a freeform pool and over the course of the week, after running it 3 times, there's not really too much it doesn't hit.