Well, my boards are still new, but I also have to take the fins out every time, sometimes several times during a session if I'm testing different fin setups. If anything, so far the rear fins are sitting better and better in the boxes and they have broken in a bit. I've put a tiny drop of oil on the screws to get a better feel for how hard I'm actually tightening them and now they work super smooth.
And the general construction of the surfin boxes is the same as FCS and the design is proven in surfboards for a long time. The screws in the surfinz are sturdier though, so it should hold up even better (and the actual boxes are stronger too).