3 weeks ago I was hit in the face by the board while foiling. I think what happened was I came off the foil & pulled away in front of it, but the board foiled after me. I must have turned around slightly & the nose of the board hit me on the side of the head close to my eye. It was cold so my face was pretty numb, but after a couple of minutes blood was pouring down the side of my face. There was a moderate cut next to my eye. The board is a 3'6" Dwarfcraft - it has quite a thick, rounded nose. I think a thin board with more of a snowboard construction would have done a lot more damage.
I think mostly it's possible to avoid impacts to anything other than your helmet, but there are occasional awkward falls that could result in a bad impact. I think a heavier alu foil is likely to be more of a problem because of the weight of the board/foil. Foiling in breaking waves increases the chances of something bad happening. The same is true even if not on a foil board ... who hasn't had the experience of coming off a SB & ending up body dragging back towards the board only to see it picked up by a wave & hurled straight at you! I can't see myself wearing a helmet with a visor though.