Only speaking of my first hand experience having built 4 different complete foils and several front and rear wings,
I offer thoughts for consideration.
I and a good friend learned on a wood core strut, alu fuse and 4mm aircraft grade alu wings in 2009.
It was loosely based on the AIR CHAIR design.
They wings were arc shaped and had very little air foil profile.
The l e of the front wing had flat bottom but a streamlined top like most naca shapes. curved tips down.
The t e also had a flat bottom but very tapered top again like most streamlined naca or similar profiles.
Same with the rear wing but of course the top was flat and the bottom was like the front top curved tips up.
For learning and low to med speed this worked just fine and is still very flyable although now with f glass wings.
Low aspect ratio used for both.
Both wings did eventually die due to LOTS of use, I believe they "work hardened" from flex and simply would not hold their curve anymore.
I would bet that 5mm or thicker aircraft alu wings, curved or bent correctly, profiled, could be made to work well and last!
For a SAME WING curved vs flat test I carefully made them very flat, they were unstable in pitch and roll, as compared
to when they were curved and yes they were still strong enough for minimal testing.
From this test I will never make flat wings, maybe a rear but then it would need a tt like fin.
I see guys and some companies using flat wings (because they are easier to make?) But as said, ,just in my flying ,curved , bent ,is notably better.
I have been lucky to have also tested other flat winged foils but have yet to ride one I like.
I see a kiteboardhydrofoil as a SYSTEM, and for it to fly well the elements need to work together, and we know so little now in my humble thinking, although the past few years have seen GREAT innovation.
It will be so fun to see what is thought of for the future!!
Just my thoughts here in early 2014.