A cable with an inadequate shield (or no shield) can act as an antenna and be a source of radiated emissions. Unshielded cables do not guarantee electromagnetic compatibility and impedance matching, and are therefore not suitable for powering VFD motors.
Similarly, it is not advisable to install single core cables, even if they are shielded, as it is necessary to compensate for inductions and respect the symmetrical configuration of the 3C+3G phases particularly in higher current applications.