Here are the disadvantages of a WBS:
- A WBS can be deceiving: It shows the tasks in no particular order, it doesn't show any dependencies, and it doesn't tell you which tasks are critical and which tasks are not.
- Sometimes it is hard to find the correct level of detail to include in the WBS: Anyone who has done a WBS in his life would agree with this point. Since the WBS has to (ideally) fit on one page, it is sometimes very difficult to find what are the tasks that should be included in the WBS and what are the tasks that should not. This often leads to the creation of a "vague" task, under which a set of non-similar tasks are grouped.
- The WBS can quickly be outdated: Although (ideally) the WBS dictates your project schedule, it (the WBS) can become quickly outdated. This is because your project schedule usually changes during the execution of the project (change requests, etc...), but your WBS remains the same. Updating the WBS is overhead that probably no project manager does (what is the point of updating a document that nobody will read).