Read through results for search "Variable ND" on this forum, and plenty of other Googling. New products and specific concerns prompt this thread.

Sensors seem to be a factor when considering variable ND, IRND, or stacking. For the purpose here I'm using a GH4 (GH5 will come soon) for video. Maybe that doesn't matter, maybe it does?

There's a fair amount of acknowledgment that variable NDs cause a loss of sharpness in telephoto lenses. I often use the 35-100 2.8 Panny at the long end and have observed as much using the Fotga variable ND. I'm looking for something better.

There appears to be a recent set of arrivals from China touting Schott glass that some find to be solid performers while others dismiss. This would be appealing if they are any good.

Pricing. I don't have an unlimited budget but am willing to spend money to solve problems (like sharpness and color cast). Spending 2x the money for 10% improvement is beyond what I'm willing to do, however.

Thanks to all who contribute here. I read a ton, comment little. I look forward to your thoughts and experiences on this topic.