So... Randy had been going very easy on me and, as it turns out, he was cooking something.
First I got an alert about a horde of sightstealers approaching. I ignored this because who would be afraid of some skinny boys. They took some time to appear, and I pretty much forgot about them. A few days after the sightstealer warning, a swarm of devourers appeared. I mobilized my mobile artillery, which was absolute garbage - it ran out of fuel without even going halfway through the map, took way too long to shoot, and missed all shot by significant margin. It did do a good job tanking devourer bites though, before getting destroyed.
Halfway through the attack, sightstealers revealed themselves, pouring from between the amphibious menace, as well as flanking from the sides. With advanced weaponry and gene-modded pawns, I was able to repel the freaks, then I went to exterminate the ones that snuck into the base proper. There were no casualties, though some people got badly wounded. Then, just as I finished bringing the wounded to the hospital, a second sightstealer horde warning appeared.
That one showed up almost immediately and was seemingly endless. When I thought I was done with them, more appeared. I'm so glad they're so flimsy. Still no dead colonists, and one freak captured to (temporarily) replace the nociosphere that served me so well before departing.