To solve the strap/rail damage issue, I always slipped a small piece of carpet (8 cm square) under the straps at the corner. I would also twist the strap as it ran across the bottom of the board so it wouldn't "flutter" and mark/damage the board.
Basically any rock hit on a horizontal surface (top or bottom) should be insignificant. However, the leading edge (nose or tail) are at the greatest risk since the hit will be straight on rather than at a parallel angle.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.