I have used Simple Green for so many years I can't remember. I have never had a problem with it but I have always used it in a spray/stream pump bottle and always diluted about 4:1.

When I am cleaning my bike and after I have finished washing the frame, wheels etc., I spray the diluted Simple Green on the cassette, chain rings, RD, FD, then brush with Park cleaning brushes the stubborn areas. As soon as I finish up I spray everything down throughly with a hose. Has always worked perfectly and makes the whole gear train look like new.

I always thought even undiluted there would be no issues, but judging from people's experiences, maybe not!