No direct experience, but I have twice found old abused bikes with busted Simplex rear derailleurs. That does not really indict them. But, on the other hand I am not sure I have seen more than a handful of broken rear derailleurs otherwise.
jim
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Cross Check Nexus7, IRO Mark V, Trek 620 Nexus7, Karate Monkey half fat, IRO Model 19 fixed, Amp Research B3, Surly 1x1 half fat fixed, and more...
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SB forever