That's interesting... I can't see why any combination wouldn't permit your Safe Area to work, unless you changed the labels in which spawns the Kill Boundary.
Typically, a Safe Area is something that you use once to cover the whole of your map. If your map is a perfect box shape and you don't want anyone going outside the boundaries, you can use either a Safe Area or Soft Safe Area to cover that box. Once a player steps out of the box they'll either die (Safe Area) or receive a return to battlefield message (Soft Safe).
Kill Areas are for maps that aren't perfect box shapes. You basically cover all of the area as if it were a perfect box with a Safe Area, and then add the Kill Boundaries inside the Safe Area that takes out chunks of the map where players shouldn't be accessing..