WBS is important for identifying tasks and showing how they relate to one another and I believe one of the issues can be not getting enough in-depth on the tasks needed for the project. If something that task turns out to be 2 or 3 than that not only changes your WBS but everything related to it. Having this happen for one of your tasks should not slow down your project by much but if this a common problem with the first set-up WBS than it can lead to major delays