I think this operator should treat all the objects as one, not as separated ones in a for loop.
See for example the file:

In this case if you Select Similar > Direction it should select all the edges of both objects. Not the "horizontal" ones in the yellow, and the "vertical" ones in the blue.

There is a mapping problem with this operator.

• Scale of comparisons should be corrected for (so area/length/direction of face gives visually correct results - accounting for object scale).
• Material mapping should be used so instead if index comparison, it matches up materials on the other mesh objects.
• Vertex group names also should be mapped between objects instead of index matches.

I think this might require refactoring the code to support this nicely, if you're confident to try this - it would be helpful,
Otherwise this might be best done by someone very familiar with the code.