Variable ND filters are not recommended very often. There's the argument that the darkening range is too small, that it's more universal with a square filter that you can put in front of any object using an adapter... And finally, there's the argument that they often have a slight color cast. Which cannot be dismissed out of hand with these. I noticed a very, very slight green tinge.
Variable ND filters were developed for filming, because the lighting conditions change frequently and can be adjusted very quickly by rotating them. The slight color cast can be quickly removed in image processing. I find these filters good for my purposes, because I can experiment with the different darkenings due to the variability, the filters are inexpensive. I bought them for my most important lenses. Not only are they good for long exposures, but also when it's very sunny. By using them, I can achieve longer exposure times. This also applies to portraits. These filters can be easily screwed into the thread and - what is really important - also unscrewed again. Anyway, I'm looking forward to the first long exposures with it.
I've also heard that the screw filters can cause vignetting. These are also easy to remove with image editing. Moreover, they are even "modern" at the moment.
The filters now accompany me in my first steps in the field of long exposures. If I feel more professional, maybe my equipment will become more perfect. Now, however, I look forward to working with them.