It's difficult to simulate the appropriate context

Many users perform tasks while performing several unrelated tasks on the same computer at the same time. This may have a strong influence on how they interract with a given product, but it is difficult to simulate.

Similarly, users may normally interact with a product at particular time of the day when they are subject to certain distractions or a certain state-of-mind. These may also have a significant effect on the task but are difficult to simulate.