some metasearch engines time out, so if any search engine site is too slow to respond its results may be excluded
with metasearch engines you do not have the same level of control over your search statement that you would have with an individual search engine. The way of phrasing a search in a metasearch engine may not be the best way of searching in all the individual search engines that the metasearch engine visits
metasearch engines quite often change the selection of search engines they use
with some (e.g. Ixquick once you have done an initial search) you can “customise” your search by including or excluding particular search engines.