Homunculus is what you want it to be. We can see existentialism in a good amount of seinen mangas, but from everything that I've read nothing compares like Yamamoto Hideo did with this piece in terms of convey, why? that depends on what you see or how much you want dig in.
The manga start with a man between two worlds, rich and poor, the only thing that put a line and separates him is Nakoshi himself ( the main character ) and his car parked between those two worlds. Everything start with trying to understand, to undercover everything, and that is the reason of the series, a man and his mind, only that.
Yamamoto Hideo put a lot effort into the meanings of human actions and the conection with the emotions that works as a brige wich sometimes leads to a tragic end. He talks also about the walls we create to hide us to the world, about the narcisism we need to live and trying to accept ourselver. It may be a sober reading, sometimes overexplained and pretentious for some, but I feel Yamamoto's effort to evoke his ideas. He has documented about human psychology and that is something that is reflected in his work very clear.
Art in this manga is stunning, with a great creativity from psichological point of view. It has cauptivate me to the end of it, with incredible symbolisms and a lot of good pages that can be culture of manga ( there are already some ). The pacing between scenes and argumental arcs is acomplished with the superb drawing of Yamamoto. Human factions and expresions, bizarre shapes with human essence, sometimes when no dialogue comes in, the art takes all the work and thats wonderfull.
Underrated manga that everyone that love philosophical and psychological themes should read or try to. A bittersweet experience that can even portrait yourself.