I watched Nekromantik 1 and 2 on video many years ago and whilst distasteful didn't find them that horrific. Although obviously we're all different and one man's meat is another man's poison (metaphorically speaking).
Likewise the scenes of animal cruelty mostly found in cannibal films are an uncomfortable watch but I can sit through them without having nightmares.
I did used to have an uncut tape of Men behind the sun though and found that horrific in the extreme, partucularly the real childs autopsy. It was one of those tapes I bought because of its challenging reputation and once I'd watched it couldn't wait to get rid of it.
I suppose one of the most challenging films in my collection these days is Irreversible. I think everyone who has been round to my gaff to watch it has been left in a state of 'shell shock'. Some people have even asked me to switch it off!