This is all Kubo's fault. He keeps trying to convey through the dialog of the manga that Unohana is the strongest shinimgami, but he ovbiously did not show that she is convincingly stronger than him. I still believe that she can keep herself from being turned to ashes just from being near him. Reiatsu can be used as a shield and can reduce the affects of physical and non-physical attacks, and she could probably continually heal herself while she is simultaneously being burned, negating its' affect. If she get's hit with zanka no tachi north, she could potentially recover from it depending on where she is hit. Zanka no tachi west wouldn't be much of a threat to her, considering that her bankai has an area of effect that could melt his flesh from the bone. The only chance Unohana has of defeating Genryusai is if her reiatsu is strong enough to get past zanka no tachi west, because if it can she can kill him. I would have said that Genryusai wins this fight if zanka no tachi north was shown to have completely obliterated his enemy, but he failed to even completely cut him in half.