The problem with schedule ahead uber xl for busy areas is the way it doesn't send the request far enough ahead of time to get the car there at the specified time. So for example - yesterday I had a car scheduled to come at 2:30. Uber assigned a car at 2:20. It got to where I was (and 5 people were waiting) at 2:45 due to traffic.
It's convenient for when there isn't time to think ahead and schedule a true car service, but just be sure to schedule the car a little ahead of when you need (and be prepared to pay waiting if needed)