I think what struck a chord was how disturbingly realistic it is(it ticks oh so many boxes seen in real child abuse cases far too cleanly). This would probably be less controversial if it were more fantastic(eg, a firm indication that the kid started it unprompted).
Well, the oneeloli genre is inherently problematic anyway (like teacher-student, most age-gap, and incest genres, among many others), and Itou Hachi has a real knack for visuals that are overwhelmingly cute without (for me, anyway) tipping over into being cloying and cutesy.
So the drawing style tends to evoke from readers an almost involuntary approval of the characters and scenes that are being presented, whatever they may be. Then when we stop to think about exactly what’s going on, the cognitive dissonance can create a kind of readerly guilt for finding something that is so disturbing in real life so adorable in fiction.
Hence the perennial cadre of people dropping into discussion forums for every oneeloli manga proclaiming, “Pedophilia is NOT OK!” as if they’re presenting new information that no one has ever considered before.