My experience with Campagnolo NR derailleurs is limited. I can, however, tell you about Simplex units. The alloy Super LJ and the 410 that PJW mentioned both were very smooth units, in my experience. And while the basic Prestige rear derailleur with the stamped cage plates had a distressing tendency to fail, I've been astounded by how many of the fancier Criterium (check me on the model, Francophiles) using the nicer cages seem to have continued doing their thing on assorted PX-10s, Gitane TdFs and the like for years and years.
Were Campagnolo products prettier? Yes. Were they more durable? Compared to the delrin ones, yes, and probably compared (long term) to the alloy ones as well. Were there far more parts and customer service and support for the Campy derailleurs? You bet. But did the Campy derailleurs shift better? Uhhhh ... no. Especially, as PJW noted, when something other than the cheapo Prestige delrin shifters were used.
Simplex, like most of the better French stuff, went bye bye because French manufacturers felt they had perfected the bicycle and sat on their laurels while the Japanese came along, looked hard at everything and improved on it. The Peugeot PY-10 Thevenet won the 1975 TdF on was an example of the very peak of design circa 1957.
Obviously, the cold medication has taken hold of me,