Hello folks!
K&F concept is a serious company. And I'll explain right away why I'm starting off so hot.
In 2024 I purchased, and I stress, purchased, not received as a sample with Amazon Vine, two K&F polarizing filters. Since it is a filter that is irreplaceable by any digital processing, I wanted to delve deeper into the quality of K&F polarizers, taking the basic Nano-k model and the top of the range, Nano-x.
Significant price difference, while remaining in the low end of the market, but surprisingly indistinguishable results.
Following my lukewarm review of the top of the range, compared to an enthusiastic one for the basic model, the company contacted me for some explanations: K&F's response was that the Nano-X promised greater scratch resistance and water repellency, but on the optical performance, they themselves admitted, between the lines, that I was right.
Well, it seems they woke up and launched an evolution of the Nano-X, which, this time, I was lucky enough to be able to try with the Vine program.
Finally, there is a difference, and it is clearly visible. The previous models had a slight, but present, tendency to turn green. Nothing tragic for the very cheap Nano-K, but decidedly annoying on the Nano-X.
Well, the green cast has disappeared. This time we are there, dear gentlemen of K&F.
The mechanics have also definitely improved. The sliding of the two rings that make up the filter is definitely more fluid than on the previous models, at least the ones I own.
The threading also seems definitely improved. With the K&F filters that I had purchased in the past, there could be some difficulty in unscrewing them, once mounted. This offers the right resistance, also to not unscrew by itself during the rotation of the polarizer, but a light force is enough to remove it easily.

So, unlike the previous model, which left me with a bit of a bad taste in my mouth regarding the optical performance and mechanical construction, this new Nano-X true color gets full marks.

P.S. just to clarify, a polarizing filter attenuates, and in some cases, eliminates reflections from non-metallic surfaces. So, in addition to eliminating reflections on glass and water, it attenuates the effect of atmospheric haze, improving the general sharpness of the photos and making the sky bluer.
But you have to know how to use it. If the orientation of the light is not at 90° to the subject or the sky, the effect may be less or non-existent.
For example, if you photograph with the sun behind you, the polarizing filter will have no effect on the color of the sky.
If you shoot with a wide-angle lens, the effect on the sky will not be uniform because the incidence of light will vary in the frame, due to the large field angle of the lens.
These are not defects of the filter you are using, but physical characteristics of all polarizing filters and you have to take them into account. In short, it is not a very easy filter to use, and often, those who do not know it, buy it and then criticize it without reason.