The competencies usually consist of a series of skills or abilities that a person needs to have to effectively perform a particular position. And, depending on how well the people performing the analyses are trained and on how much data, whether objective or subjective, that exists about a position, the more accurate the competency profiles are. Do they really discriminate between positions. In my own experience, most competency profiles are too generic to be as useful as they could be for recruiters. The best lists contain very specific skills required to do that job, as opposed to some other job.
Useful competencies are very specific and very much tied to a particular job.
The danger is that those skills may not be those that are really needed to excel. Many job profiles, especially in the high tech world, can become obsolete very quickly. Can it be determined in a screening process whether or not a candidate has these competencies. In many cases, competencies might include things like good team skills, or ability to deal with customers in a friendly manner. Subjective competencies are difficult to discern and may only be known after the candidate is on the job.