Even though I managed to accomplish the tasks I needed, the time it took me to accomplish them and the foreign approach to the UI made me not happy. Implicit requirements are still requirements.Is the time it takes to accomplish a task really an "implicit requirement"? I think not. It's one of the primary metrics we use to ensure usability, and hence a standard nonfunctional requirement.
I have little doubt that most product managers neglect this metric in their requirements documents. But that neglect isn't due to the requirement being "implicit". It's due to the general tendency of product managers to pay excessive attention to alleged functional requirements that are in fact functional designs, to the exclusion of nonfunctional requirements that drive them.