I recoil from the idea that the view of southern slavery as unmitigated horror needs correction. Meanwhile, I don't have to defend every sentence in Nehemiah Adams, because the context in which Yarvin chose to cite him (approvingly, as one of his favorites) is as a rebuttal to the idea that slavery was harmful to blacks. If someone's being unfair to Adams in this situation, it's Yarvin.
I think, if anything, we tend to underestimate how southern slavery has continued to contribute to horrors that still continue well over a century after its abolition: