I think I mostly agree with you, ILTB. Folks argue about a a couple of percentage points of efficiency as though they're the whole world, when in fact a good derailleur system will vary by 2% just from gear to gear. My beloved Rohloff appears to average about 2% less efficiency than a good derailleur set-up, but that's unimportant to me. It might be important to someone else though, and the OP did ask for potential negatives...