I'm in the process of rebeding all the deck hardware on Brigid: drilling oversize holes, cutting out core material from the sides of the holes with a sharpened allen wrench on the drill, and refilling the holes and undercuts with thickened epoxy. When I redrill for the hardware, it will be through epoxy and any leaks shouldn't reach the core.
I'm going to all this trouble because I've removed too much balsa mush from several small and large deck and cabin top repairs. Previous owners just didn't maintain the hardware fittings.
I can understand balsa's appeal to the builders: cheap, light, oddly strong, and when grain is placed vertically, relatively resistant to water penetration. But over time, any delamination in the presence of leaks, and the stuff completely fails.
My larger repairs have been with marine plywood presealed with epoxy, the smaller repairs have been with thickened resin.
I'll avoid balsa altogether.
--Dave