Unified Search is a bit different from Federated Search in that it involves issuing a single search query against a consolidated or unified data source.
The benefits of the Unified Search approach is generally faster search, better relevance and a better search experience. Since the Unified Search is only querying a single index, the performance is better than searching the multiple systems that you have to do with Federated Search. With Unified Search, the search relevance can be applied across the entire unified data set. This means that the results will always be ranked by relevance and are not dependent on the relevance of the data source. Finally, Unified Search produces a better search experience since search results will be blended from all sources and facets, filtering, sorting and other key search UX features are built on top of the unified index.