A variable ND can produce a similar exposure effect to a fixed ND, and its biggest advantage is convenience: you can dial in the strength you need instead of carrying multiple filters. That can be very useful for things like waterfalls and other long-exposure work.

However, variable NDs often have trade-offs compared with fixed NDs. Because they work using a polarizing effect, they can make skies uneven, with some areas darker than others. They may also introduce color casts, and cheaper models can show dark bands or other artifacts, especially at very wide angles and near their maximum density.

A fixed ND is generally more uniform and predictable, since it’s meant to reduce light evenly without the extra polarizing side effects. Also, in practice you may not need infinitely variable control: shutter speed can often be fine-tuned with small aperture or ISO changes.

So: yes, a variable ND can work well, especially as a flexible, lower-cost way to experiment. But if you want the most consistent image quality and even darkening, a good fixed ND is usually the safer choice.