Good performance fins, and very light. But too expensive, and too fragile. They need a lot of sanding to fit many finboxes too.
Actually, “too expensive and too fragile” is a criticism that applies to a lot of SUP fins these days.
The fit is a problem too: FCSII fins are a nightmare really, because if they don’t fit well (which they won’t, in many boards), there isn’t a lot you can do about it.