By language count Niger-Congo is the largest family in the world, with something near fifteen hundred members covering most of sub-Saharan Africa. Its best known subgroup is Bantu, several hundred closely related languages including Swahili, Zulu, Shona and Kikongo, whose spread across the southern half of the continent over the last three thousand years is one of the largest single expansions in human history and is not in dispute. What is in dispute is the rest.
The classification comes from Joseph Greenberg, whose 1963 book The Languages of Africa reorganised the field by mass comparison of vocabulary, sweeping away a previous scheme built on racial categories. Greenberg's grouping has held up better than most of what it replaced, and it was also never demonstrated by the comparative method in the way Indo-European was.
Specialists including Tom Guldemann now argue that the sound correspondences and reconstructed morphology required to prove the family have simply not been produced, and that Mande, Ubangian and parts of Atlantic sit in it on weak grounds. The atlas records Niger-Congo as a family with its unity marked debated, because that is what the state of the evidence is. That is not a demotion. It is a statement about which parts of a very large claim have been proved and which have been assumed, and the distinction is exactly the kind a reference work exists to keep.