This is somewhat misleading. PowerFlarm doesn't actually let you "see" mode C and S transponders. It warns you if a transponder equipped aircraft is in your vicinity. It tells you the altitude and an approximate range, but you have absolutely no idea if the aircraft is in front of you, behind, you etc.

If the PowerFlarm guys had bothered to implement TIS-B, it would be be able to tell you exactly where the transponder equipped aircraft is. Similarly, you get absolutely no visibility for UAT equipped aircraft or drones.

This may be an acceptable product for Europe, but definitely a half baked solution for the US market.