I can confirm the X as you approach max with the Lightcraft Workshop variable ND. My variable ND also has a dark spot on the top right corner of the frame which may not be present in all, but that problem isn't the only problem I have with the variable ND. My recommendation is to just buy a set of fixed at the largest filter size and then use some step up rings for your other lenses.
The set of Tiffen's I mentioned above are 77mm (since my Canon L lenses are 77mm). I use step downs to fit my 72mm and 58mm filter threads. The only drawback is that the hood then doesn't fit. So, I have two (cheap) flexible hoods with 77mm threads (one for wides and another for normal) that work quite well.
The variable ND is nice for its flexibility and the results can be fine - also have a 77mm one. The footage from the following video used a variable ND for all of the footage from 2:12 to 5:20.
Sorry - I tried posting the link without the huge video window, but couldn't.Weekend at the Lake on Vimeo
Don't bother with the rest of the video, as the video was more of a personal project). The footage in that time is from a 90 minute boat ride, with changing light, shot with the Canon 24mm 1.4L and a T3i/600D. I was able to ride the variable ND to manage exposure using my desired aperture. Given that we were on a boat and I didn't want to carry anything other than the camera, changing the ND would have been nearly impossible. So, variable ND's have their utility. In a set shot, I wouldn't use it in favor of fixed ND's. For flexibility, I'm glad I have one in my kit.