Most of the low reviews for this series come from people who seem to not even have watched it. You can tell this, because instead of offering actual insight regarding its content, they fall back on played-out "arguments," pointing out that the first US slaves were sold by other Africans--as if those people had a hand in US policy for the next 400 years.
Or, the letter ostensibly signed by 4 historians, arguing inaccuracy with a claim the creator made regarding the Revolutionary War. Again, even if she was incorrect in that assertion--which she admittedly overstated and subsequently revised--it does not change the fact that slavery was, in fact, a major component to the US economy, and it doesnt necessarily discredit the claims of the rest of the series. This is a point that was made in a Politico article by Leslie M. Harris, who strenuously argued against the Revolutionary War statement and who noted that the letter was "just as misleading" in some of its own claims.
The actual series itself is informative, engaging, and entertaining; however, given its content, the knee-jerk opposition it faces from the racist, the oblivious, and the downright stupid is wholly unsurprising.