A variable ND works by stacking two polarizing elements and rotating one relative to the other. As their alignment changes, less light passes through, so the filter becomes darker.

Because of that design, variable NDs are fundamentally based on polarization. Simpler versions may behave much like using two polarizers together, so polarizing side effects can occur. More refined designs use circular-polarizer-style construction with quarter-wave plates on the outside and linear polarizing layers inside. That reduces the usual scene-dependent polarizer behavior and avoids issues with some camera metering/AF systems.

So, in practice: a variable ND is not a completely different technology from a polarizer—it is built from polarizing components. Better designs minimize obvious polarizer effects, but they do not eliminate all compromises.

A common side effect is the "X" or cross pattern, especially at stronger ND settings. This artifact tends to become more noticeable as the filter is set darker, and can also be more visible in some shooting situations.