Awesome Art: Believe it or not, the attention to detail is amazing, one may not get it due the Only Six Faces trope, but the clothes, cities, weapons, attention to the accesories, skin tones, etc. Make the simplistic faces contrast a lot with the really detailed pages.
Critical Research Failure: Mustard gas is a blister agent, but the chemical attack in volume 2 has symptoms similar to a nerve agent or a pulmonary agent. Even the most rudimentary knowledge of chemical warfare, or WWI, or Unit 731 should have prevented this mistake. In-story, one might wonder who the hell uses mustard gas to knock out a dog, since mustard-gas is slow-working...
There's more problems. The vegetation growing in cities is problematic in appearance, mostly because plants tend to overgrow everything fairly quickly if not kept short. And 'everything' means that trees can grow on top of a building if there's a possibility for that. However, as plants require light, they will not grow inside buildings unless natural light can reach them. The roads overgrowing with grass are another problem. Why aren't there clumps of asphalt visible between the patches of grass like on a real overgrown road? Why don't the characters stumble while walking on such a road, because it is nigh impossible to see where the road is destroyed and where it isn't?
Also, the characters in the early chapters seem remarkably clueless with regards to survival in the wilderness without any possibility for support from civilization..