And yet an exhausted sasuke could tank a strike from Naruto being smashed into a rock wall, at 12 sasuke could handle naruto's clone barrage slamming him into the large cliff repeatedly no problem, sasuke also survived a close encounter with indra's arrow and NE boosted two rasenshurikens with one of them also being a bijudama and sasuke still survived even if susano protected him he still took damage, and the war legend Madara is, you know he's taken some hits.
You talk about them having the weakest body but you're forgetting one thing, THEY EACH GET PIERCED IN THE HEART! Sjeesh, the heart is at the center of the body just a little bit to your right, you can just feel it pumping on your left. Madara got stabbed in the heart, sasuke got stabbed in the heart, and izuna got stabbed in the heart, it's a human weak point, vital point. Heck even when kabuto was healing sasuke it was near that center area around the heart. Not many Naruto characters can heal a direct pierce through the heart, in fact if you're not a senju or anybody with insane healing prowess, you are not taking a direct stab in the heart, you're going to die. Uchiha's are actually very durable but they don't have healing or body sturdiness like the senju have. Because of the whole yin-yang thing.