The first thing I would like to say about this manga is that I feel it is very difficult to give it a numerical rating. Up until now, my rating has been based upon a mixture of my level of enjoyment and the level of success that the series has in doing what it intends to do, be that tell a story, put across a message etc. For this reason though I have given an overall score with this review, I would like you to ignore it and just read what I have written.
Kuzu no Honkai is a whole different kettle of fish from the average series. To me, Kuzu no Honkai was not "enjoyable", I did not like the choices that the characters made, it frustrated me to see them sink to deeper levels, and furthermore to cause others too feel despair as well. However, it never sought to be a tale that viewers could relate perfectly to, it is a tale of our main characters choices when faced with an unrequited love and the repercussions of those choices. It fulfils its intentions perfectly, showing true consequences of this unpleasant underbelly of love where people will do whatever they can in an attempt to achieve that which is out of reach.
I think many negative reviews are based merely on the fact that it is a difficult manga to find pleasant to read, with people overlooking the fact that it does in fact do everything that it set out to do in the first place very well. i would certainly recommend it, there may be others like it, but it is the first time I have come across something like it. The art style is also very impressive, something I did like about the series a lot.
Finally, the follow on Scums wish Decor, brings a slightly more pleasant conclusion to the series, with it taking place a number of years later, looking in on the way the lives of the characters we have seen progressed and where the events of the main series brought them to. So even if you did not like the unresolved state in which the main story leaves you, hopefully this will be a better outcome in your opinion. .