I dont like concave, I find it odd and annoying having to "point your feet" forward (well, okay, to leeward....)
Both on hydrofoils and surfboards.
Especially if you stand with rear foot leeward and front foot windward (as I often do) - it feels even more wrong with concave... (to me)
Flat is great, but a little, maybe some, convex really pleasant too sometimes.
Say when you heal over like when having some kitepull.