The main problem with the current classification scheme for personality disorders is that: A. it falsely implies that nearly everyone has at least one personality disorder B. the criteria for diagnosis are so detailed and specific that even extremely disturbed people fail to meet them. C. the categories often overlap, making diagnosis unreliable. D. it contains too few categories to be useful.

First, it's important to understand the meaning of each given option before identifying the correct answer. Here's a brief explanation of each option: A. it falsely implies that nearly everyone has at least one personality disorder - This option suggests that the classification system is too inclusive, and it tends to overdiagnose a personality disorder in a large part of the population. B. the criteria for diagnosis are so detailed and specific that even extremely disturbed people fail to meet them - This option implies that the classification system may be too rigid in its criteria, leaving out some who need help but fail to meet the specific criteria. C. the categories often overlap, making diagnosis unreliable - This option states that the various categories used in the classification system are not well-defined and distinct, leading to inconsistencies and unreliability in the diagnosis of personality disorders. D. it contains too few categories to be useful - This option suggests that the current classification system may not be comprehensive enough, lacking enough categories to describe a wide variety of personality disorders.

Based on our understanding of the options, we can eliminate options A and D - as they seem less concerned with the system's reliability and more connected to its inclusiveness (A) and comprehensiveness (D). These are not necessarily issues that could impact the classification scheme's overall validity.

Between options B and C, option C provides a more significant drawback that could undermine the reliability of the classification system. Having overlapping categories can lead to confusion and inconsistency in diagnosing personality disorders, which can cause problems in treatment and research. Thus, the correct answer is: C. the categories often overlap, making diagnosis unreliable.