I actually finished Monster a few days ago but I will state my feelings on it for posterity's sake: I think Monster, while composed of a number of well written and evocative vignettes, ultimately fails to execute its overall emotional arc well. If the story had been explicitly framed episodically with an overarching B-plot connecting these episodes, I might feel more positively towards it, but the fact that it must nominally develop its "main plot" each chapter while devoting most of its screen time to tangentially related stock characters was extremely frustrating. None of the twists or reveals actually felt consequential because information about the main plot is fed to you so painstakingly and in a way that's so repetitive that its conclusion is largely predictable and almost anticlimactic.

Monster ultimately does not do a very good job of developing any of its main cast. Its story and characters warp around Tenma, who is just not very interesting. I was frankly just fed up with him and the unilateral adoration that every character holds for him by the end of the story. Nina was extremely underdeveloped for the character she should have been and supposedly was within the story and any pathos that could have been elicited by her was neutered by first having her be barely present for the vast majority of the story, and second by also relegating her into being a Guy Who Just Loves Tenma. Eva was probably the closest any character had to a real character arc and her writing was just so absurdly misogynistic for nearly the entire story I couldn't enjoy any time she was on screen. Johan should have been an interesting character, but the manga got so long and drawn out that by the end I was just fed up with him.

The story overall was weirdly saccharine in a way that felt almost childish in contrast to its intended tone, and while that's not in itself a bad thing, its attempts to portray humanity as Overall Good felt simplistic and often cheap. It kinda goes hand in hand with how horribly it attempts to address WWII. "Well, The Nazis Felt Really Bad About It" and "If You Think About It, Japanese People Were The Real Victims" are not, uh. Themes that show a willingness to contend with complex morality or any real belief in human goodness aside from superficial self-reassurance. Also Urasawa Naoki is insanely insanely insanely afraid of sex.

Ultimately I think Monster would have been way, way better if it had just been shorter and had treated its main plot in any reasonable manner. It had a lot of interesting, evocative moments, and a lot of the ideas of the main plot could have been really good to chew on if the main plot hadn't lost momentum long before it was actually wrapped up. Nina and Johan are given very interesting treatment on occasion, and I have been thinking about their characters and relationship quite a bit since finishing Monster, but the plot was so bogged down in the Various Extraneous Dipshits and the need to conform every aspect of Nina's character around Tenma that there's no actual interesting conclusion to be drawn from them.

I think on an arc-by-arc level, Monster is quite well written, and it has a few really strong moments. It just makes such poor use of its format overall that it sours the whole experience.