What is scary to look at is pretty much determined on an individual level. There are people who find the xenomorphs in Aliens to be tame but at the same time find the large-eyed depictions of Grays (a type of alien associated with abductions) from films like Communion to be horrifying, and vice-versa.
As someone who watches pretty much every horror and sci-fi horror film imaginable I can only remember feeling an unsettling sense one time and that was during a few of the scenes during Fire in the Sky when Travis Walton is abducted and being dissected. You're only given rather quick, tense shots of the aliens and the machinery they are using but all the fast movement and shock reveals are pretty effective, and there's certainly something unsettling about having other lifeforms treat us like we do lab rats.
I definitely think that with horror, 1940's horror movie directors rightly understood that less is more, because the more you see the monster the more desensitised you become to its presence, much like violence itself. If you have to reveal the monster, save it for some pivotal moment in the story like the climax otherwise you've just got another Freddy Krueger or Jason Vorhees tramping about killing people by the dozen and camping it up.