All very good points. But using higher density XPS doesn't solve the issue of outgassing. There was a camper build on this site where the builder experienced delamination due to heat (first time the camper saw the sun). It was sanded (only) 60 psi XPS. Outgassing of XPS at elevated temperatures can cause this. I've heard of one surfboard builder who perforates the FG skin on his boards with tiny needles. But I suspect that delamination of this sort can be eliminated by ensuring a good bond.
XPS should not outgas. It only aborbs 0.1%-0.3% moisture by volume because all cells are completely closed. Gas does not circulate freely withing XPS foam like it does in EPS foam. Gas can only be released by the thin layer of XPS surface cells if they rupture -- crushing or tearing. When using EPS, vent are often used to adjust internal pressure when the air between beads expands with heat.
However, the XPS panel will expand and contract with extremes of heating and cooling -- influenced by density. This can be enough to cause the FG skin to separate from the foam.
One pro board builder made an XPS board sanding to only 150 or finer, used a thin coat of epoxy to seal the foam and then laminated. Said the board lasted for years without delams.
Those who use XPS for board builds protect those boards from heating (XPS softens with high heat -- in or on top of a car). Upper limit of service temp is 165 F.
Also it would be wise to use light color pigment for XPS skins (e.g. white) to reflect rather than absorb solar energy.
Perforation is used to improve the bond and reduce surface skin movements. Hydroflex surfboards patented their perforation tech.
My next build for body boards will use 25 psi OC 250. I will perforate using a WPT with 2x standard tyne number and sand to 150 grit. I will use specific perforation (lengthwise) slit orientations relative to flex axis. I may use opaque white pigment in the bottom epoxy. The deck will be covered with 1/4" EVA which should proved some insulation from heat. But the modified WPT and slit orientation will be my main method to improve bonding.