Those old Simplex derailleurs shift fine when correctly set up and, when correctly set up they just automatically shift into low gear if the cable breaks. The catastrophic flaw is that if you drop the bike on the right side, all its weight goes on the derailleur for a moment, and the derailleur cannot deflect. A normal high parallelogram, like a Gran Sport, just moved out of the way as if you were pulling its cable. The result is that rigid arm derailleurs like Sinplex, Huret, Cyclo, etc, often get bent, sending them into the spokes eventually. They are judged by how they die, which is rather unfair.