I understand how gimbal lock is a serious risk in case of CNG where the axis of rotation change in operation, and as they become coplanar you lose degrees of freedom, the respective axis duplicating given degree of freedom and losing another.
I still don't see how this could be a case with three reaction wheels installed perpendicular to each other. They always remain perpendicular, and the axis of rotation of the craft will never be parallel to more than one. Where, in what situation does gimbal lock occur with this sort of attitude control?