What are the limitations of quantative methods for effective performance appraisal in terms of HR Performance management?

Quantitative assessment is the Ranking system: The appraisal for a particular period is mainly based on the job assignment where he has many other exceptional capabilities which he may  not be in a position to exhibit.  When good number of employees are involved, the ranking and grading system may mainly concentrate on the performance actually undergone, as the intrinsic strength in the employee may require qualitative considerations and some critical approach deviations to the normal procedures i.e. qualitative assessment is also very important to analyze the hidden capabilities and creativity.