I think it's the "maternity ward scene" that really pushes people over the edge.
While body horror, pregnancy (as birth and death) and slaughter of innocents is pretty common in Japanese Hentai or Anime animation, its considered obscene and taboo in the west, even in a fictional setting.
But overall as an amatuer film-maker and former media students I can say while later ALIEN and PREDATOR sequels lacked the holistic quality of the originals, technically they are as good if not better in the make-up and VFX effects department.
Also I take even films like AVP-R scene by scene and I can say there is some overlooked quality in the film-making, including atmosphere, suspense, editing and (gulp) even the acting is better than some people think.