What a questionnaire is for
An operational due diligence questionnaire is often treated as the review itself. Completed, filed, and taken as evidence that operational risk has been assessed. That is a misunderstanding of what the document can do.
Its real function is narrower and more useful. It structures a conversation so that nothing important is left to chance, it creates a written record of what a manager asserted and when, and — most valuably — it identifies the small number of answers that need to be pursued somewhere other than the questionnaire. A good questionnaire tells the reviewer where to look next.