The rate gyro signals (x, y, z and composite) are just measuring spin rate in °/s around those respective axes - these are not physical gyros for stabilization. The rate gyro data, together with the 3-axis accelerometer data, are used by the IMU via the derived quaternion components to calculate the instantaneous pitch, roll and yaw of the aircraft. I primarily look at the pitch, roll and yaw values rather than the rate gyros but, as you noticed, they will both show the onset and rate of spin and you can use either.
Pitch/roll/yaw clearly show the signature of propulsion loss, and should even show the difference between a lost prop and a failed ESC, since if the prop flies off then it traps its angular momentum, whereas if the ESC seizes it will transfer the angular momentum to the body of the aircraft - those two cases will result in similar immediate pitch and roll but different yaw response. The motor speed data provide more unambiguous evidence, since if the prop flies off then the motor speed rapidly increases, whereas if the ESC fails the motor will essentially stop.
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! Anyway, I'm off to research accelerometers (as I know nothing about them). -CF
