The identity of the boss who is weak to Blanka's Tropical Hazard is this. In canon, this person (it's Rose, by the way) is already confirmed to have low blood pressure. Considering that watermelons lower blood pressure, the possible risk of hypotension might explain why said character isn't too keen on being pelted in the face by a watermelon in the first place.
The bosses' pattern are rather erratic, especially that of Ryu's, what with spamming hadoukens and invincibility-granting spinning kicks. But then again, don't most Street Fighter players do so?
Remember how in Mega Man X and Mega Man X2, X learns how to perform the Hadōken and the Shōryūken? Now take this game into consideration where Mega Man gets the former from Ryu after defeating him. Suddenly it starts to make sense, as if Dr. Light studied the weapon after the events of the game and later had it made as a weapon for X in 21XX.
might explain why said character isn't too keen on being pelted in the face by a watermelon in the first place.