What is wrong with variable NDs?


Senior Member
I've spent a pretty penny replacing variable ND filters over the years and I've come to the conclusion that they are just bad investment.
No matter how much you spend on these, they just don't perform well over time.
For the cheap ones, the degradation to the image is almost immediate. Color shfts over different ND values is pandemic. Loss of sharpness, Coma - just to name a few issues.
The expensive ones are not much different after a few months of use. Sharpness fades and sometimes dust gets between the layers.
Strangely, even the ones sitting unused in a drawer also degrades.
These sort of "aging" doesn't seem to affect fixed ND filters though. I have Cokins from decade ago, and they are still pristine.
Anyone has any observations contrary to mine?