This is one of the films where I can most easily understand the
complaints of the folks who hate it while not at all sharing them.
Just a couple examples (I don't want to make this too long):
(1) issues of a coherent or "logically connected" plot. It's very easy to see why that is brought up if one is expecting anything like a typical American film, as Fulci retains characters and scenes that are not going to pay off much later, many things in the plot simply remain unexplained, and the film gradually changes subgenres many times. But I do not watch this film expecting anything typical. I see it more as the film version of a nightmare, and all of those things make perfect sense in the context of a nightmare rather than something following screenwriting "rules".
(2) Also, I think a lot of folks have preconceptions about this as a zombie film that are not fulfilled. Partially because of some of the things I noted above (like the film traversing a number of subgenres and not "worrying" whether it sticks with any of them), it's easy to see how those expectations are thwarted. However, I try my best to remove all my preconceptions before I watch something (it's not possible to totally do that, in my opinion, but I try to get as close as I can), and part of that involves me avoiding as much information as I can about a film prior to my first viewing. So the first time I saw this, I had no idea that anyone considered it a zombie film, and I wasn't expecting anything resembling other zombie films I had seen. Because of that, I didn't consider it at all a zombie film myself until I was past the halfway point at least. I just enjoyed it for what it was--the many things it was--as it unfolded.
. . . as I noted, those are just a couple examples of the typical complaints I can easily understand but that I do not share. There are many others. I think it also doesn't help that Fulci is considered one of the "sacred cows" of Italian horror, and this is one of his better-regarded films. It builds up expectations in anyone watching it for the first time now or in recent years that are going to be very tough to meet.
I love the film. If you haven't seen it yet or you did and you didn't like it and want to give it another chance, maybe try watching it without preconceptions of what it should be doing, what it should be like--especially ideas that it should be like other films you're familiar with, and just go with the flow of it as if it's something your mind is presenting while you're dreaming.
http://www.rateyourmusic.com/~JrnlofEddieDeezenStudies