The prime advantage of the Simplex Delrin components was that they were relatively light and inexpensive. They needed no break-in period, unlike many other derailleurs, They were smooth out of the box. On the other hand they required abnormally high effort to shift. The levers were quite flexible and the prime source of the high effort. The push rod front derailleur was arguably the worst front derailleur of the boom era. The rear derailleur was the best piece and was a competent performer, when mated to better levers. However, as stated they did wear and get sloppy. They also did break but rarely when new. If they were misadjusted and went into the spokes, you could pull the pins out of the pivot housings, but they were actually quite tough. However, despite manufacturers claims, the delrin did appear to get brittle with age and environmental exposure. After a few years, they did start to come back, in pieces.

If you want to keep the bicycle original, the aging aspect is a concern. If it does break, even if you find a NOS replacement, the aging is still a concern. On the other hand, most LBS have bins of used Simplex parts, usually at very reasonable prices, The shifting can be improved substantially with different levers and a HyperGlide compatible freewheel.