"Plastic" covers a broad spectrum. The Simplex Derailleurs were made of DuPont Delrin, which is an acetal resin that is still being manufactured (albeit not as derailleurs). I had a Simplex Criterium on my early 1970s Gitane Tour de France that shifted quite well. However, my bike shop days taught me that Simplex derailleurs tended to break-down with time whereas their Campy NR counterparts tended to break-in.