If the user has no idea of where to look on the Web outside of a search tool, it is difficult. If the information or the type of information is more obscure, it is difficult. Because we tend to see a lot of questions on a wide range of subjects. And we know how to look beyond the obvious. The Invisible Web is like a great reference area.
Experience is the key.
And work is the way. Most any search tool can find that by symbol. But what about the exchange of the currency used in India or Jordon or Mexico. It is not easy navigating the current guides to the Invisible Web. Because it depends on subject directories.