The ND filter industry can be confusing - there are a lot of misconceptions about what they do or do not (sometimes even put out by companies who sell them!), and a lot of talk about quality without any quantitative information.
The quality of ND filters can vary dramatically, with some cheaper filter causing color changes or even degrading the image quality.
Some filters do not have any coatings - and they can be very, very hard to clean once you get fingerprints or grease on them. One place you don't want fingerprints or marks in on the piece of glass in front of your camera!
When we create a product, we focus our energies on making it as high quality as possible. If the glass or isn't good enough for a professional photographer, we don't use it.
To be able to perform testing, we own a scanning spectrophotometer - it measures the quality of glass samples by testing different wavelengths of light passing through it to see how the glass affects each of these wavelengths. We have tested almost every brand of filter that we can find, and we are continually surprised by how little actual quality has to do with marketing hype!
To show an example of this, we tested a filter from a competitor who makes some of the more expensive filters in the market - these retail for at least double what Camera Butter filters cost:
This is plot of how cleanly each color is passed through the filter. The vertical axis is how much light is allowed through, and the horizontal axis is the wavelength (i.e. color).
Neutral density filters are supposed to be just that: Neutral. In other words, they should pass all colors as equally as possible. The flatter the line in the above chart, the less the filter is messing with the colors in your video.
The turquoise line is a very well-known competitor, who has a reputation for high quality (and also high prices). The orange line is a Camera Butter filter. You can see that the competitor's filter lets almost 3 times as much red through as it does purple. Although you can fix the white balance using your video editing software, you shouldn't have to. Plus, when you have to make corrections like this, you lose dynamic range and detail in the color gradients.