Engines that search a Web site were not as useful as the researchers expected. About a third of users availed themselves of on-site searches as a primary means of navigation. Others resorted to searching only when they were unable to find information by navigating the site.
I recall seeing a presentation by Spool at a symposium, in which he referred to in-site searching as the "navigation of last resort," to which users turn only if everything else has failed. Most often, users tried a single search, briefly scanned the results, then bailed (even if the item they wanted to find was among the search results).
It was also found that search engines aren't as helpful as expected. In some instances, users who used a search engine took longer to accomplish a task, or were ultimately unable to do so, while users who relied on navigation were able to complete the task more quickly and more often.
The main problem with search engines is that they return a long list of results. Splitting out search results into separate pages to shorten the list was marginally worse than giving users a single, long page with all results. None of the users tested understood how to use the "find" command to search within the results screens.
Search engine results often provide insufficient information about each "matching" page to indicate the content of the page. Some contained page titles only, others contained a few sentences of page content, but none of it was found to be useful.
None of the users could explain the way that search engine results were organized, or the reason that any given item was considered to be relevant to their search query.
A search engine can also become a "tar pit" - the user gets lost after searching, and cannot navigate back to the home page, or any other page that would be helpful in finding their way to the information they seek.
To make search more precise, some sites add Boolean logic and additional search parameters. This causes more problems than it cures: most users do not understand the various features, do not take the time to use them, and often misuse them in a way that results in a dead-end.
Another presentation - studies have shown that users (still) do not fathom boolean logic in searches, and are more often frustrated by the ability to "search within results" when the original search yielded too many matches.
Also, there are instances in which a search engine returns redundant pages from various sections of a site, pages that contain similar information, pages translated into other languages, and other such useless results that frustrated users and further buried any useful information.
Another presentation - in-site search can be detrimental. The presenter provided some usage statistics of e-commerce sites in which revenues were higher and abandonment rate was lower in sites after removal of an on-site search.