The key flaw with using critical path method for scheduling is that planners (e.g. project managers) misuse it. Let's see what Wikipedia says about it:
"Originally, the critical path method considered only logical dependencies among terminal elements."The problem arises when planners try to convert a critical path diagram (CPD) directly into a schedule by superimposing a timeline on the diagram. This kind of CPD may yield a best-case schedule, but projects of substance almost never progress accordingly.
First, you can't anticipate with any level of certainty the effort and time required to complete tasks. For example, how long will it take the user interface programmers to finish the UI for a software product? You can only make an educated guess.
Second, you can't easily and accurately model the collaboration among different members of the team. An effective product development effort requires a great deal of interaction among the product manager, developers, testers, and even marcom and sales. Tasks and roles tend to blend together much more than a CPD can convey.
Third, you can't anticipate the requirements discoveries that will take place. It might be that prospective customers say, "Oh yeah, I forgot to tell you about this other problem I have." Or it might be that a competitor releases a product that renders your product obsolete unless you significantly enhance it. These requirements discoveries will, in turn, render your CPD obsolete.
Fourth, you can't predict the logistical and architectural challenges that will arise. You may find a critical bug in the software tools you're using to develop the product. Or a subcontractor may suddenly go out of business.
Fifth, you can't predict the integration problems that will occur. You eventually have to integrate the deliverables resulting from the disparate tasks. It is difficult to anticipate how long it will take to integrate these pieces.
Understanding the anticipated dependencies using critical path analysis can be useful for identifying what went wrong. However, a CPD on a timeline does not allow for nimble adjustments. Nimble adjustments require an agile approach to scheduling.