Adjustable ND Filter (or variable ND) with Micro Four Thirds question...

this brand:

http://www.bhphotovideo.com/c/product/758957-REG/Fader_Filters_HD_VND_77_77mm_HD_Variable_Neutral.html

and another which is also multicoated, etc.  I did get color shifts toward yellow.  Also, the sharpness was affected.  I also tested it on my Nikon DX camera that I had at the time and though similar results were there they weren't as bad, except on a very long (400mm) focal length.  I remember that I used a tripod for all tests.  Shorter focal lengths were not affected as much.

I like the concept of variable ND, but I had to return the item for reasons mentioned and I have been using good multicoated ND filters of different sizes since.  Could it have been the fact that I used a 77mm size and used step up rings?  I have read rave reviews about many of these variable ND filter, and, as I mentioned, I want to like them.  I am thinking of trying a different brand.

Thanks for all the replies!