There is absolutely nothing wrong about Parasite.
The anime keeps you on edge from start to end...it has no flaws as a story-line, art or flow...the rest depends on the personal taste of its person.
As an anime, it is exceptionally intriguing, there are no unnecessary scenes or dialogues and every sentence that is uttered has meaning.
The characters are interesting, although not so influencing as to get obsessed with them.
It is also rare, the fact that it poses questions of the world's general ideology. Why is the human species superior? Why is the "survival of the fittest" being denied by humans as soon as someone fitter appears, from the moment they established it?
The few battles that it has are conducted in a most zestful and alluring way, while dubstep that accompanies them makes you all the more excited...neh, chibi?...
If one was to find flaws, that would be at the appeal of the story. Even though, it is particularly well made and thought out, it emits an invisible coldness that makes it somewhat distant. For example, One Piece is an anime which makes you bond with it because it focuses very very much on the characters. Someone might dislike the fact that parasite focuses too much on the story and uses characters to narrate it.
If One Piece's characters are on the surface, Parasite's are underneath.
That, along with the violence and bloody scenes. However, unlike other anime with raw violence, Parasite doesn't use scenes as a tool to show blood, they feel almost necessary, hence the reason one might not mind them. They do not provoke, compared to other which simply want to show blood, you can say that Parasite has to.
