I like to tease some folks about NR derailleurs but the NR works well over a closely spaced freewheel that does not exceed 24 teeth... it was a very long serving design and the things never wear out and like Berto said, it shifts poorly but it will shift poorly forever.

Many racing derailleurs come with the same limitations for range and will behave poorly when you try to exceed the derailleurs capacity... will say that I am partial to the Suntour Cyclone and Superbe rear derailleurs as I do not believe there has ever been a better designed friction derailleur than these.