In reading the first twenty pages, it has become clear why it was banned.
Extremely graphic descriptions of acts which I have trouble describing.
The main character appears to have worked with Dr. Mengele, but did things a thousand times worse. And the author feels a need to give it all in intricate detail.
The part I've read describes Lord Horror's island and how he has huge gardens and strung up in the trees are dead bodies he has wired up as electrical transformers to power his radio station, which can be heard across the world. And then talks about what he does with body parts of female Jews and whatnot.
The only book I've read that comes close in terms of graphicness is American Psycho by Brett Easton Ellis. But in that book, you knew the guy did these things because he was insane. Lord Horror does these things because he likes it. It's somehow worse.