This one is better on cloudy days.
I have several polarizing filters from K & F. I think they have made me a better photographer.
It's been snowing lately. I wanted to take photos of the untouched snow. The sun came out and the shine was reflected in the snow. This filter toned down the glare and gave much better shots.
Now, without this filter and my black K&F filter, I never head out to the day.

Snow is too deep for field testing right now. Indoor testing is very good. Easy and straight forward to use controls. Steady signals for each type of metal. Construction is light but sturdy. Will help a lot with my arthritic hands and other joints. Can’t wait for the snow to melt and start using outdoors.

I like this camera it comes with a case.

It’s is portable, water proof.

ND filters cause the image to "darken" in order to be able to film at medium exposure rates or larger apertures.
The Osmo Action is of course not a film camera, so I wouldn't say here that NDs are a must - but if you want to film primarily in the SLOG profiles, want to experiment with long exposure (even in the light), try time-lapse recordings or in the snow sun is on the way, he should think about it.

You definitely can't go wrong with the K&F Concept Set. The filters are of incredibly high quality and come in a custom-made case. The case can be closed securely and is flat enough to be stowed in a backpack.
The filters can be changed quickly, which is important because of course you need a different one in the twilight than in the midday sun.

Incidentally, a highlight is the anti-reflective filter, which filters out reflections in the snow or on the water surface, for example. You really get a new perspective with him, which is fun.

got these for my Mini 3 (not the pro), after I accidentally got snow in one of the motors. Fit great, and easy to put on.

Good area to film outside in the bright (keyword: 180° rule), even in snow or in midsummer (here ND1-8 would be clearly too little)
Hardly any vignetting on the edge
Fixed stop at 8 and 128, in between points for better/more repeatable adjustment
So far, the filter makes a scratch-resistant impression
Easy screwing and unscrewing
Good case to store the filter quickly and safely
Fits well with the Sigma 24-70 2.8 (protective cap can be placed on filter)

I haven't had the time to set this all up yet, maybe this upcoming week in the 50s with no rain or snow in the forecast. But its nicely packaged and the description matches the product perfectly. Excited to finally install this.