When I finally found a useful search pattern that returned a result (yes, singular), the response to navigation was HTTP 404.
Count me as skeptical. YaCy might though fulfill the niche of searching a homogeneous set of mildly cooperative sites, i.e. specialized and customizable search for nerds.
Imagine if YouTube and Vimeo standardized on the YaCy protocol instead of (or in addition to) the in-house HTTP+JSON/XML mess of a search protocol Google uses for video search, and whatever protocol Vimeo uses, if any stable at all. Then virtually all video sites would have to implement it in a compatible manner, allowing authors to make video playlisters gathering videos from any of multitude of providers (with the user having the theoretical option of adding their own).