Alternate theory: The human brain is reacting to unfamiliarity and not alien features. We strongly associate Uncanny Valley with things not-quite human but it’s my thinking that it’s a tribal thing. Nowadays we see a ton of faces of all variations but I bet when we were hunter gatherers, we only saw features of our own tribe. The moment you meet another tribe, I’d bet this response is to create fear of the unrecognized human. It’s also probably there as a punishment mechanism for us seeing faces in everything.

The times that the uncanny effect hit hardest is when you think something is human or is a face potentially before finding out you’re wrong. So that’s my basis for thinking its there to keep us from being mistaken.