...no, not all variable ND filters have the X factor. I have the Heliopan 6 stops version and have not seen this effect.
I respectfully disagree that some variable ND filters lack the 'Maltese cross' artifact. They will all have it, just to varying degrees. The B+W reportedly has a darker maximum (they claim 11 stops) than other variND filters, meaning you won't see the artifact until you get to very dark settings.
There are two factors which increase the visibility of the 'X' - the density to which set the filter, but most importantly, the angle of view of the lens it's used on. At ultrawide angles and darker settings, you'll see the artifact. With better filters, you only see it at the widest angles and darkest settings.
FWIW, here's an image from a customer review of the Heliopan variND on B&H
He doesn't indicate the lens used, but based on the fact that there's also mechanical vignetting due to the thickness of the filter, it's probably an ultrawide lens.
Bottom line, if you plan to use it with only moderately wide angle or narrower lenses, a good quality variable ND filter works well. I'd define 'good quality' as B+W, Singh-Ray, Heliopan, and Tiffen (Tiffen isn't known for high quality filters, but their variND is an exception). For example, shooting portraits with flash outdoors (where you need ND for a wide enough aperture at Xsync or lower to overpower the sun). If you plan to use it for long exposures with a wide or ultrawide lens, I'd go with standard ND filters of varying strengths (or maybe go straight to the 10-stop!).