Disadvantage of PERT

1. Data Quality

Your PERT chart breaks your project into independent activities. It requires accurate and consistent data to establish resources, relationships and deadlines. If you hadn't built a computer before, you might not be able to break down the activities cleanly, and you may have problems getting precise data for complex projects as well.
For example, it is hard for project teams to give accurate time estimates for activities that are new to them. If some of your team members aren't familiar with installing certain hardware in a computer, then the timeline may end up based on opinions rather than accurate estimates. Even if you're confident that you control internal data, external factors may cause problems. If a supplier does not deliver a key computer part on time, you may need to modify your chart.

While PERT encourages you to focus on the critical path, you may run into problems if you ignore activities on other paths. These might grow in importance over time, especially if there are delays or problems that then affect the critical path.
For example, if an activity to install a piece of computer hardware on a shorter path overruns, you may have to put your critical path on hold until you get back on track, which extends your timeline. If that part doesn't get installed quickly and causes the deadline to extend a lot, it may become your new critical path, and you may need to modify your project analysis.