The Breakthrough filters are screw on only, not the 100mm resin type slide in filters. so less convenient than say Lee filters. The Breakthrough filters are nice and neutral, and their step up rings are nice. I got mine from the Kickstarter and it was not a pleasant experience, fortunately now the founder is not dealing with the buyers as far as I know.
That being said, I have not finished all my testing but I am finding that NISI filters, which are a Lee type filter holder and filters are excellent. As neutral as any filter I have tested. My testing has been real world and with a spectrophotometer. The Nisi filters are very neutral, much much more than the Lee filters.
For screw in filters the Breakthrough are very neutral, B+W not so neutral.
Alan