I wanted to like this a lot more than I actually did.
My first sign of trouble was the much vaunted "the underground railroad is an actual railroad!" scene (I'm assuming that is not a spoiler since it is blatantly stated in every single synopsis, though it might have been a better scene if it wasn't?). It smacked of magical realism. Generally, I want to smack magical realism, so I should probably not be surprised that this didn't work for me, but I was still hoping to get some sense of awe from it. I didn't.
Overall, that was my real problem with the book: I didn't feel awe. I didn't feel much of anything. I've had more visceral reactions to textbooks about this time period. One of the "main" characters disappears partway through the book and it's somehow no big deal.
Our main character, Cora, has basically no emotional response to anything that happens throughout the novel. Some extremely horrific stuff happens throughout the novel. It is a novel about slavery. It is a novel about the racism that was common at the time period even of people who were not slave owners. There are murder and executions and beatings and horrible things happening. Cora remains alarmingly aloof from it all.
I'm almost inclined to think that this is somehow an intentional choice, that she is there to serve as author mouthpiece throughout. She has occasional bizarre knowledge (at one point she casually mentions how difficult life is in Ireland at the time period) and while she makes excellent points on several occasions, she makes narrator-style points comparing white life to slavery life aloud to moderately racist people, who never seem to respond. It feels like she's talking to the readers instead of the characters she's interacting with.
Maybe that is all more magical realism--I'll be completely honest and admit I don't get the genre at all--but it didn't work for me. I wanted to be right there with her in this brutal time period, and not only was I not there, she didn't seem to be there most of the time. Everything remained clinical throughout, and with a story like this, set in a time period where everything was horrifying, I was hoping for more.