It would have been effortless for Gyasi to have created something trite. There are many novels about the slave trade, and many novels about experiences in both Africa and America as a result of that.
First, Gyasi creates the right kind of structure - a genealogical one, which allows her to tell multiple stories across multiple generations. Then she adds one more layer to that by telling both the stories of those who remained in Africa and those who were shipped to America, and putting them side by side throughout the novel.
Don't miss the many opportunities to understand what the African experience was like (which in many stories was brand new to me), and don't miss the fresh perspectives that Gyasi brings to the American experience. Her writing is stellar; so are her insights.
It's likely you'll get to the final chapter and be ready to scoff at her conclusion, but you will know better because of the strength of her writing to that point. She may have created a somewhat mystical ending, but that downplays the power of her craft. It's not at all mystical; it's the unveiling of truth.