Final answer:
The main disadvantage of middle-digit filing is that it's complex, requires extensive training, and can lead to errors and inefficiencies in file retrieval.
Step-by-step explanation:
The disadvantage of middle-digit filing is that it may require extensive training for staff to understand and implement this filing system efficiently. It can be time-consuming and confusing to sort through files when the middle number is used as the primary sorting feature, especially for new employees or those not familiar with the method. Additionally, this complex system might not be as intuitive compared to other filing systems, which can lead to errors and inefficiencies in file retrieval.