These variable ND filters are made of 2 polarizers. When both are parallel, there is a minimal darkening due to the combined density of each. And when you rotate one vs. the other, the amount of light transmitted decreases and tends to zero when the polarizers are perpendicular.
An elegant solution at first glance. But because of their polarizing effect, this will have an impact onto the scenery: in certain areas they will be "enhanced" like by any PL filter, on other areas they wont.
Plus, generally speaking combining 2 filters means 4 air/glass interfaces, thus problems of diffraction (image tends to become slightly hazy).
Plus, a PL filter adds a faint color cast. Two superimopsed filters add more.
And finally, these cross-type filters are made of acrylic resins which are not as good as optical glass. And the cheaper ones may be made of not perfectly planar parts, and not (multi)coated.
For all these reasons true stills photographers don't use this system and prefer real ND, to keep a high picture quality.
You could also consider that if this is critical for hi-res pictures made by DSLRS it is not of such importance for video, which is very "low res" compared to stills.
I (as a photographer using high end DSLRs) wouldn't recommend this kind of cross-type filters, and do not use them. But again: I'm a photographer, not that much in video. So, just my 2 cts.
