I hear you, but that does not change the physical characteristics of the materials.
Secondary bonding (mostly what your doing when rebuilding a transom) is SO much better with epoxy that you almost cant screw it up. When the boat was made you had very fresh resin (that was the same kind as used in the mold for the skin), its as good a scenario as possible.
Please put this into perspective, the difference in cost to use the material that WILL ABSOLUTELY work is around $110 (difference in 5 gallons poly at 30/gal and epoxy at 50). That is just one days fuel bill for a boat that should last you decades if done properly.
Do what you wish, just make sure you understand the compromises your making.
You can always use poly to make parts, then bond them in with epoxy. Nothing at all wrong with that.
Good luck
