Unless you are a video focused person, Variable ND filters are not worth it. For the most part, still photography doesn't demand control over ND strength to the nth degree. One to two +/- stop variation isn't going to matter much. Furthermore, you are dealing with some issues that are part of the variable ND filter design; which is simply two polarizing filters back to back. Issues that come with color casts, "X" pattern, two layers of glass in front of the optic, and inconsistencies across the frame when used at ultra wide focal lengths. There is also the expense. A single "high quality" Variable is going to cost more than high quality 1 or 2 ND filters of different strengths.... I also highly suspect the quality of the polarizing filters used to make variables filters.
If you want to experiment, simply take two polarizing filters and mount them back to back then rotate them at various degrees apart. I did such experiment and was not happy with the results. Yes.. I used quality filters in the experiment.
I use a 3 stop and 10 stop ND filter with a series of step up rings to fit all my lenses. 3 stop for handheld day light with fast apertures and 10 stop for tripod mounted long exposures. I never felt the need for much more.
Again, this is for still photography. Videographers often find Variable ND filters absolutely necessary because the constraints to apertures and frame exposure time that need to remain consistent even in changing conditions.