The drivetrain on my wife's bike will spin backwards for about 3 full crank revolutions. The drivetrain on my CX bike will do about 3/4 of a revolution. Rear derailleur, 100%. She's got a 105, I've got a clutched Rival 1 with a whole lot more spring tension. If the cassette spins freely and the cranks spin freely, you know there's nothing mechanically wrong. It is something easy to get hung up on though, I think.