Excessive drift, same with temperature changes, all of which are dealt with by firmware of the gyro.

Short version, it drives them wonky. I'm guessing the ET either doesn't have any internal dampening, or the software isn't sophisticated enough to deal with the vibration of larger IC engines.

Gyro mounting in a helicopter should always be done in the area of least vibration, though some will argue otherwise, nearest the CG makes no difference to the gyro, vibration however is a meance to them.