Depends on which derailleur. While the SuperLJ is often regarded as even better than the Campagnolo Super Record, this of course isn't true for the cheapest bottom-of-the-line derailleurs they sold - some of them are indeed quite bad.

But apart from that, as problematic as the Delrin plastic may be (especially when it ages), back then it was a great invention in lightweight building. E.g. the Simplex Criterium is a quite nice derailleur. Basically, the Delrin plastic idea is the same as with carbon bicycles nowadays. Biggest problem was, derailleurs with plastic parts didn't look that good, and a lot of people disliked them. Yet, the quality was quite good, and I wonder how many carbon bicycles from today will still be around in 40 years...