A drawback of using Internal Control Questionnaires (ICQs) is that they can be less efficient than conducting observations and inspections when many control procedures need to be covered. ICQs can be time-consuming to complete and may not provide the depth of understanding that direct observation and inspection can achieve. They often require follow-up to clarify responses, which can further increase the time and resources needed to obtain the necessary assurance.
Reference:
The Institute of Internal Auditors (IIA) Practice Guide on "Audit Evidence Collection" IIA Standard 2310 - Identifying Information: "Internal auditors must identify sufficient, reliable, relevant, and useful information to achieve the engagement's objectives."