One drawback is that skipping the search results display breaks the user's mental model of how search works (from John Ferrara's Search Behavior Patterns):
- User types in a search
- Search engine gives back matching results
- User reads the results and picks the best one
Breaking that model is not necessarily a bad thing, but you'll want to make sure the user perceives what you've done (returned the actual page rather than a search engine results page) and why (because there is only one matching result).
Further, I think you'll want to give users options to break out of the single result if it did not match their intention. Healthy search behaviors include (from Peter Morville's Behavior Patterns collection, and his book Search Patterns):
- Quit - can be because of success, or because of failure and the user has given up.
- Narrow - constraining a search that returned too many results
- Expand - broadening a search that returned too few results
- Pearl Growing - given a good result, finding more like it
- Pogosticking - bouncing between the search results and the individual pages returned. Some pogosticking is healthy - users are sampling the results.
The design you describe seems to support only the Quit behavior. Assuming the search was successful in helping the user reach their goal, that's fine. Otherwise, the user is dumped on a page irrelevant to their needs, possibly with no clear path back.
You can accommodate a single-result strategy, and still support other healthy behaviors. For example, by explaining that the search returned only one result, and that the page itself is displayed, users are less likely to become disoriented. By providing opportunity to refine their search, you allow the user to re-target an errant search: