I one of my mission I saw something strange: Crysalids were unable to enter 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 Crysalids)

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.