Variable ND filters are made up of two polarizing filters stacked on top of each other. The angle of polarization between the two is what determines the density of the system.
Polarizing filters don't work so great on wide angle lenses because those filters have the most effect when the subject (sky?) is 90° to the sun. Since a wide angle lens covers much more than 90°, the sky is darkest at that angle and then gets lighter away from that angle. The effect can be very unsatisfactory, to say the least. Regular ND filters do not have this effect. However, since variable ND filters are composed of two polarizing filters, they will have the same effect with wide angle lenses, though you may feel free to use any regular ND filter on any wide angle lens of your choice.