In his latest blog post, Aaron Tay provides an insightful and thoughtful piece on the emergence of open scholarly discovery indexes.
In 2015, Marshall Breeding the guru of Library technology and discovery summarized the state of Library Discovery in a NISO White paper entitled "The Future of Library Discovery"
He described the state of play at the time, in which index-based discovery services were "entirely dominated by commercial providers" - listing the usual suspects of Summon (owned by Proquest), Primo (Exlibris before acquisition by Proquest), Ebsco Discovery Service and Worldcat Discovery Service.
In a fascinating section entitled "Open Access Global Discovery Service or Index", he discussed the possibility of "the creation of new index-based discovery services based on open source software and
an open access index".
Lamenting that while open alternatives for other aspects of Library technology such as open source discovery interfaces, Library Services Platforms, Knowledge Bases were starting to emerge, the possibility of a "open access" central index did not seem likely to be in the future.
His analysis of the difficulties particularly in terms of technical complexity still probably holds though in the four years since he wrote this, yet we see the rise of many new discovery indexes that are build wholly or partly (e.g. Dimensions) on open data drawn from many sources - chief among them Crossref and Microsoft Academic Graph(MAG)...
Lens.org is the best by far for precision searching
Taken together, a search engine that rates highly on the two mentioned tests would give the searcher a very controlled and precise search.
I was totally not surprised that Lens.org came up on top with nearly perfect scores in Jeroen's test on filter features.