Critiquing PERT

You decide to introduce the concept of PERT to a business who is struggling with project scheduling. One of the managers of the company tells you the following:

"Well, we thought about PERT before actually. The best part about PERT is that it recognizes uncertainty in project time estimation as it is - there is no attempt to sweep it below the carpet, and pretend that it does not exist. On the contrary, PERT gives a rough approximation of the uncertainty in the final completion time. Of course, the simplifying assumptions made during the analysis do lead to an underestimation of the probability of long project completion times.

But, as with everything else in life, PERT too is not without its problems. While going through the process of analyzing project and activity completion times, you have to make quite a few simplistic assumptions. We have to assume, for example, that the critical path does not change. However, it is possible that the critical path that was identified based on the most likely or expected completion time will not necessarily end up being the critical path. Scenarios in which another path takes longer than the identified critical path may be ignored. There might be a tendency to understate the expected completion time, and this will definitely underestimate the probability of late completion. And that is why we don't consider using PERT."

Critique the manager's claim. (Do you agree or disagree with the manager's claim? Why or why not? Can you give any examples of how he is right/wrong? If you believe that underestimation can happen, is it a big problem? Do you think that a non-changing critical path is bad? Why or why not?)