Just finished putting together a Peugeot with the plastic Simplex derailleurs. Getting the rear to work was the worst multispeed experience since my Varsity in the '70s. The bike is a 12 speed, getting all 6 sprockets to work reliably took a lot of tweaking. No B screw? One of the few times I really needed it. I suspect the stem shift levers don't have quite enough throw for the job as well.
I won't even go into the front derailleur sinking and twisting at every attempted shift. It's a clamp-on that is about 1/2 a millimeter too big. Waxed the frame while the bike was apart making it too slippery to hold. A beer can shim and a dab of silicone did the trick.