I have had a Quad for three months and used it loads due to an excellent November in the UK. Certainly this "is" a definite but minor issue. On the back boxes and fins do work loose. I eventually slipped the fins into the boxes with some sail cloth to make them a very tight fit, which is what one used to do with fins back in the old days to tighten them up. This worked perfectly and has stopped me having to tighten the screws continually (and worrying I was going to eventually strip the treads!!). I leave the fins in the board all the time never take them out.
This works fine, as I say its a minor teething issue and there are bound to be those with revolutionary new designs........its an amazing board totally made for the real world wave sailors just like the EVO was.