I one of my mission I met a strange thing -- Cryssalids 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. (Thus my only survivor -- heavily wounded sniper were able to kill 6 Cryssalids)

So are there any other limitations?

E.g. I guess Sectopods can't use stairs/drains and never seen a Berserker to use one.