The usual complaints I see are that characters have a lack of "emotion" and 'reaction". Subtle facial features and pent up emotions do not mean there is a lack of emotion.

In real life, when someone is happy they do not jump around in joy and their faces do not suddenly get larger with a gigantic smile. These are over exaggerated emotions. When someone is angry or upset, not all people just collapse in tears or pop blood vessels. Most pent this emotion up and most people do not notice it unless they are looking very closely.

Though Bokurano's premise is out of this world, everything is rather human.
This manga is fantastic, placing the characters in a hopeless and desperate situations. It forces the raw emotions out of each character and the reader themselves.

If you are into deep stories that challenge the fundamental nature of life and humans, please read this. Do not read this if you are looking for a funny or happy go lucky story. You will not find it here.