Although I do see how could Conrad’s work be perceived as biased, principally due to what we would now consider as ethnocentrism (anthropology was still not a consolidated field at the time), I think Heart of Darkness is arguably one of the most powerful books written. Marlow’s descent into hell is representative of the rotten aspects human soul that let itself be seen during war.
What he is showcasing through the hellish narrative isn’t, as authors like Achebe believed, a racist manifesto. Instead, what Conrad is trying to do is depict the degrees of barbarism that humans are capable of. This barbarism and insanity plays against the mind of the colonialist.
I do think that Achebe does right in showcasing the thriving Nigerian culture as a place of light and vivid expression rather than one of darkness, but I don’t think Conrad’s darkness is linked with Africa at all. Here at hand is the notion of the darkness of the human soul. Kurtz’s wretchedness and the madness of many of the colonists is evidence for this. Even with Conrad’s depictions of the Thames and Belgium, for instance, as a rather dark and sepulchral place are also signs that he isn’t exactly siding with the Europeans.
In short, I think Conrad’s argument is Modernist in its nature. After visiting the Congo himself, he could eyewitness the depths to which humanity can sink into. What he captures in the book is that stench of death, the sights of misery and the monstrosity of violence.