Most of my criticisms of book 5 also apply here. This book REALLY pushes the boundaries of the readers engagement and sympathy. There's more Pegasus Collective that I now usually brush over as they just repeat the same inane philosophy. I'm not sure if these segments are really meant to be serious, or some comedic relief, or what. They just seem scatterbrained, uninteresting, and fairly generic hippie nonsense.
In addition Punpun straight up becomes a very bad person. He's miserable, he's rude, he assaults Aiko again, he murders someone, then attacks a stranger, is just a complete asshole all the time. It's certainly interesting to take your unlikable protaganist and make him even worse. I continue to find the story interesting, and the characters engaging, but not as much as the first 4 books.
I find myself overall liking the manga less and less over the last 2 books. It continues to be more and more miserable, outlandish and strange - without any of the lighter balance. The earlier books had a lovely balance of pain with small moments of pleasure or nostalgia. It's always been a painful and awkward journey, but these last two books are really a slog through misery. I still respect and admire what it's doing, it's bold choices, it's strangeness. And although I respect it still the same, I think I enjoy it a little less. Let's see what the finale brings. Probably more pain.