Usually, no—variable ND filters are not generally comparable to a good fixed ND filter. Because they work by rotating two polarizing elements, they can introduce side effects such as uneven darkening, cross-polarization artifacts, and color shifts. Those issues can vary across the frame, which makes them harder to correct than the uniform cast you might get from a fixed ND filter.

That said, quality varies a lot by brand and price. Better variable ND filters can perform much better, with more neutral color and better sharpness, but they tend to be expensive. So the tradeoff is convenience versus optical consistency: variable ND filters are flexible and fast to adjust, while fixed ND filters usually give more predictable, uniform results.