Having to step in the right spot to spawn zombies in is the problem. You're on rails the entire time and if you step off the rails the POI Quest is essentially broken.Funny how many people complain about the dungeon POIs taking choice/exploration out of the matter, but as soon as a POI doesn't hold your hand all the way through people also complain.
If, for example, a zombie breaks a locked door and you enter the now broken door there's a chance you'll miss the trigger for an event, leaving you running around the entire POI trying to figure out what you missed.