Anyone who makes it through the first book will see that the series starts to figure out what it wants to be here.
Don't get me wrong; awful shit still exists in this book. Rape, violence, and slavery are still brought up constantly. The ways people react to them, however, is incredibly emblematic of what I believe Fujimoto's themes are. What will people do to survive when they're pushed to the brink? What batshit morally corrupt actions will decent people allow once it becomes socially acceptable? And is it good to be knowingly complacent in that?
The introduction of Togata's character to the story adds a deliciously meta flavor to it. They're entirely self-aware, and beyond pure storytelling, they're pretty damn funny, too. And yet, the more they interact with the people in this universe, the more the reader finds out how these characters think and why they're doing what they are.
Those who are (rightfully) uncomfortable with the subject matter should probably not read this series, but for those people who can stomach it, the narrative begins to get so much stronger the deeper you get. Fujimoto starts to get a clear understanding of what he wants to do starting here.