In one of my missions I saw something strange: Chryssalids were unable to enter a small truck's trunk with just one panel broken. They even didn't attack a person standing right in front of this panel. (My only survivor, a heavily wounded sniper, was able to kill 6 Chryssalids.)

Why couldn't they attack me here? Are there any other limitations?

I also think Sectopods can't use stairs/drainpipes, and I've never seen a Berserker use one either.

EDIT: I was wrong, Sectopods can climb ladders (or jump?). Otherwise I have no idea how next screenshot can happen as at previous turn he was downstairs.