I though his endurance was actually pretty impressive taking a lot of damage from Law alone, then going on to face Luffy twice in g4 it's not too bad
Law's attack tore apart his organs, doffy just had to stich his organs back together, had it been a ryou, fire, lightning or anything that can destroy rather than tear apart, based attack it wouldn't have worked
For instance, if ace landed a clean hit on doffy like law did with gamma knife, doffy would be down, can't stitch back together dust