It's complicated. It depends on the sensor and the motion of the object. Because the gravity vector doesn't change spatially very quickly if you are doing loops you can use the accelerometers to calibrate the gyro biases in the sensor fusion loop (via the gravity vector). You can also use the magnitude of the vector to calibrate the accelerometers (roughly). Nothing helps pin all that stuff down like a global position measurement though. Some sensors are just less prone to bias drift as well.