A varible density ND filter is essentially two circular polarizing filters. You should only use variable NDs within their recommended 2 to 8 stop range.

If you go beyond the “MAX”, you may experience an uneven exposure or color shift that appears as an “X bar” in the image.

This is more common when using higher densities on full frame cameras and at shorter focal lengths especially with lesser quality filters; however, it can be experienced with any variable ND filter on any camera and at any any focal length. Lesser quality filters may only allow a 5 stop range before a pronounced "x" bar appears.

This is a common issue– it’s the law of physics when combining two polarizing filters; however, it can be eliminated.

First, adjust your focal length and then reduce the density (stop value) setting by rotating the filter ring until the color shift disappears.

Rather than spend $$$$ I've built my own vari-ND filters with two CPL's. It is quite easy and as long as you are mindful of the above limitations they work quite well.