I'm not sure what difficulty it was being played on but when I played on normal, the Aliens weren't shy about raping me. They were extremely intelligent at a distance, using cover to move around, using the shadows to flank me. On more than one occasion when I hadn't taken them out in a distance they surprised me by attacking me and when I'd turn to take them on, they'd retreated into the shadows again.
I found the AI very effective. Especially when there's about 3 of them. I got killed more than once.