Maintenance...
My first commuter had something jam in the chain and rip the derailleur off during a pedal stroke. My second commuter had the derailleur cables and other parts corrode during the winter of commuting. The cable broke one day. Then I went SS. After that bike got stolen I went FG. Now I have a FG road bike (Kilo WT), a SS MTB for the winter, and a SS folding bike for errands and other odd jobs. My wife's winter commuter just had the cables corrode and she had to ride it around SS for a day. After that she is thinking of converting it to SS this winter. You just can't screw something up that is so simple.