In a reply to the criticism of his classification of the languages of the Americas, Greenberg (1989: 107) characterized his work on African languages as follows:
[...] my classification is clearly the basis of present-day African historical linguistics. There is no alternative classification, and the few proposals cutting across my four basic groups have received no general support. All disputes have been at the level of subgroupings, some of which I had said were tentative; and indeed some of the changes were suggested by me. When one considers that even today there is no unanimity regarding Balto-Slavic as a subgroup of Indo-European (IE), similar disputes among the much less studied African languages should come as no surprise.
Is this still a more or less accurate description of the status of his classification of African languages? I.e., has his broad classification been refined and received relatively minor improvements regarding subgroupings, or have there been major revisions or serious questions about the validity of (parts of) his classification?
Greenberg, Joseph H. 1989. “Classification of American Indian Languages: A Reply to Campbell.” Language 65 (1): 107–114. http://www.jstor.org/stable/414844