http://www.thingiverse.com/thing:44579 is a good example of a box that has a top which needs bridging. The bridging works fine on Makerbot and Cura, but on Simplify 3D it insists on drawing the perimeter lines before considering bridging. This means the bridging is in thin air and simply does not work.
Also, when modified design to have thicker walls, one side of the fill is absent, i.e. three sided area being filled. However, in some cases, the bridging is not side to side from the supported wall to the supported wall, but diagonal from a supported wall to a strange forming the new edge. Surely the fill direction logic needs to consider if the side in question is supported or not, and try to pick fills direction that aims for properly supported ends to all of the bridge lines.