XPS is “closed cell” foam. It absorbs very little water (virtually no water). The gas bubbles in the foam are very small and closed (no interconnecting air passages like EPS has). The only way gas can be released is if the cells are ruptured. Even after sanding, the surface is not porous, just a thin surface layer of very small ruptured cells. As result the epoxy has very little surface area to bond with and virtually no foam penetration (no soaking in like in EPS). This is why those who use it rough the surface before glassing with 40-60 grit sandpaper – increases bonding surface area. Poking holes in the foam does little to increase surface area for bonding.
Foam strength/shear for either foam will be related to density (pcf) and minimum compressive strength. In general, low density foams typically have low minimum compressive strength. The lower the minimum compressive strength the easier it is to damage the foam (pressure dings, delams, etc.).
XPS is mostly polystyrene but has a small amount of additives. Currently DOW is adding nitriles to the mix. In the 80s they were adding polyethylene. Not much bonds to polyethylene. To see what additives and how much are in the foam, check the MSDS sheet for your foam.