Not all Simplex rear derailleurs are unreliable, and all of them work quite nicely when new. But Simplex was fond of Delrin plastic, and many of their low-end derailleurs used Delrin in the parallelogram body. Grit eventually erodes the pivots and makes the derailleur action sloppy. But there are Simplex derailleurs with all-metal bodies as well, and these are more durable than the Delrin models.

No. When new, they work quite well. The upper and lower pivots are both sprung, which allows the pulley cage to track the cogs well. Shimano copied this design for their derailleurs as well. SunTour's design fame came from their patented "slant parallelogram," which mounted the parallelogram on a slant with a nearly horizontal alignment. This followed the cogs even better than the double-sprung pivots. It's interesting to note that when SunTour's patent expired, Shimano quickly incorporated the slant parallelogram into their own derailleurs, and almost all modern derailleurs have both double sprung pivots and a slant parallelogram.