My initial impressions of the Breakthrough X100 Holder were very good. However, after bit of use I found that the thumbscrews work loose.
In addition, the screws that hold the red fixings in place are disconcertingly loose so there’s a bit of rattle. These can only be tightened with a precision screwdriver.
Another issue with the holder is the lens aperture is a little large for the square filters. As a result, even when the filter is carefully aligned in the holder, there’s a small gap at the top or bottom (or both) of the filter.
Also, the foam around Breakthrough’s strong ND filters doesn’t make contact with the adapter ring and this plus the small gap could lead to light leaks that may cause problems with images – especially those shot using very strong neutral density filters to enable long exposures in bright conditions.