1. "Perfect answers" that relate to the topic at hand. For example, Findjango has an index of Django's documentation and lists related documentation at the top (above search results). For a large percentage of searches, those links will answer the search immediately (also, the links deeplink to the correct location on the documentation page, as opposed to simply dropping you at the page's beginning).
2. Because of a narrow scope, it is possible for a vertical search to be more current.
3. Because of the narrow scope, it is possible to apply custom relevancy algorithms suited to the specific niche which may outperform generic relevancy algorithms. Findjango has begun traveling this route, but is clearly not there yet.
4. The ability to collect and display targeted metadata. Although I'm not taking advantage of it yet, Findjango knows the number of followers and last commit date for each GitHub repository, and can display them in-line in the SERP. For a targeted audience this can be worthwhile to add, whereas for a main-stream search it would represented a negligible usecase.
5. I believe that vertical search engines can be monetized much more effectively--and while doing so create more added value for the user--than a generic search engine. Certainly I have no numbers to back this up as of yet.
In the end, my argument is that a vertical search can outperform a general search engine within its vertical. It is certainly not my argument that it will inherently outperform, and certainly Findjango lacks the breadth of content to compete thus far.
This is a long-term project, certainly not an overnight road to success.