Ehret 2001 (or see Nostratic.ru) looks at first sight like an astonishingly detailed reconstruction of Nilo-Saharan, with nice binary splits and loads of technology-related words for archeologists and anthropologists to sink their teeth into. Why shouldn't specialists take advantage of this amazing opportunity to correlate historical developments to linguistic ones?
I just found a handy answer to that question. Bender (1997:175ff) gives the 15 cognate sets in Ehret 2001 that are represented in the most sub-families of Nilo-Saharan. 3 of the 15 look distinctly like Arabic loans.
1387 *wàs “to grow large”: Fur wassiye “wide” and Songhay wásà “to be wide” are both from Arabic wāsi`- واسع. The other items cited – Ik “stand”, Kanuri “yawn”, Kunama “increase, augment”, and Uduk “to tassel, of corn” – are scarcely obvious candidates for being related to one another in the first place.
1297 *là:l “to call out (to someone)”: Kanuri làn “to abuse, curse” and Songhay láalí “to curse” are obviously from Arabic la`an- لعن; Kunama lal- “to denigrate” might be from the same source. That only leaves Uduk “to persuade, incite to do something” and Proto-Central-Sudanic “to call out”.
718 *t̪íwm “to finish, complete”: almost certainly Songhay tímmè “to be finished”, very likely Uduk t̪ím “to finish”, Ocolo t̪um “to finish”, and maybe even Fur time “total”, are from Arabic tamm- تمّ (impf. -timm-), as Bender (ibid:177) considers probable. That leaves Proto-Central-Sudanic, Kunama, and Maba “all”, Kanuri “ideophone of dying animal” (!), and Proto-Kuliak “buttocks”. The “all” set looks rather promising – the whole etymology, not so much.
There are plenty of other Arabic loanwords in Ehret's “Proto-Nilo-Saharan” – a particularly egregious example is Kanuri zàmzàmíyɑ̀ “leather bottle-shaped water vessel for journeys” (#1223 *zɛ̀m “to become damp, moist”), and other especially clear-cut cases include #1173 < sawṭ, #1185 < šamm – but the fact that they include a significant proportion of the best cognate sets is what really strikes me. If a reconstruction attempt can't distinguish a widely distributed recent loan from a cognate set that split more than eleven thousand years ago, any information it gives about readily diffused items like technologies is completely unreliable. For another review from a similar perspective, try Blench 2000 (not sure why it appeared a year before the book's nominal publication date...)
The more I read about Nilo-Saharan, the less convinced I am that it exists (much less that Songhay belongs to it.) That means the classification of the languages of quite a lot of Africa is basically up for grabs. It would be great to have a reexamination of the area.