When you conduct a usability evaluation of a non-trivial website or product, most likely you will only find and report the tip of the iceberg — some 30 random problems out of hundreds.Rolf Molich
The CUE studies consistently show that the number of usability problems in most real-world websites is huge. Most CUE studies found more than 200 different usability problems for a single state-of-the-art website. About half of them were rated serious or critical.
The essential truth behind Nielsen’s recommendation to test with 5 representative users is its achievability in terms of resource and scale. Also, that usability testing is undertaken as part of iterative cycles of user-centred research and design.