I had one, and it worked well. I had to send it back as it rusted, but I think they corrected this issue.

The trick is to take time to adjust before first session the screw on the side to just be able to operate the lever, but without any significant wobbles. And always operate the lever with the fin wet, as the mechanism is lubricated by water.

The designer told me that the gap between the fin and the hull is not an issue, the water layers close to the hull are so turbulent that basically nothing in the first 4 mm induces significant drag.

I am sure it is very useful for high-level prone shortboarders, but the limited travel range of the system on boards as big as SUPs makes for noticeable, but subtle effects. If you feel that your center FCS or Future box is already in a nearly ideal position, I would warmly recommend a shifter. The small adjustments will then make noticeable difference in handling.

But since I personally needed more range (My production Gong Alley 8'1" distance between the fronts of the side and rear fins is 28cm but I would have preferred 22cm) I decided instead to reshape my fins, or in my latest custom to have an US Box installed with a Quobba center fin on a FCS adapter + finjak, for a travel range of 10 cm.

PS: Warning: when moving the fin on the water you can lose it if you are not careful, as the the fin can come off the inset.