As the K&F Concept 72mm True Color ND2-400 Slim Variable ND Neutral Density Filter is, in truth, still a budget filter, I have rated it 5 stars despite its obvious issue, common to many VND filters, of an X being visible when the maximum density is exceeded (not a literal X, but a cross shaped variation of light and dark) although, from my observation, this wasn't as pronounced as my low-cost budget version! (but this was under studio lights, not the bright, sunny, day I wanted)

The adjustment ring can be turned continuously, with no stops at minimum & maximum, so it is easy to exceed the maximum density and have the distorted cross shape (I believe the 2 elements of the filter are polarised, so the pattern appears at almost 90 degree difference) appears, and may not be noticed until the shot is taken. (don't ask...)

The photos above were taken:
No filter
Filter at minimum
Filter exceeding maximum - where uneven lighting of the frame is clearly visible.

If the weather improves later in the week, I'll add some outdoor shots.

Examining the RAW images in Lightroom, there are minor colour differences between no filter and filter, but so low as to be accepted as possibly just illumination difference.