BTW It is stated in the official justsu book that It's as hot as the sun. It burns the users vision and that's why their eyes bleed. Also in Japanese myth when Izanagi washed his face after escaping the underworld his left eye became the Sun Goddess Amaterasu. Soooo makes sense considering how he based most of the Uchiha's clans techniques off of this legend....
The databook hypes up a lot of techniques for the sake of dramatic effect:
- Earth Release: Earth Spear - "With the chakra flowing through all parts of one's body, the skin is hardened, increasing its defence power to its utmost limits. The amount of techniques that can break through the areas hardened by the Earth Spear are close to zero, not including its undeniable weak point: Lightning Release ninjutsu."
- Summoning: Triple Rashōmon - Being able to use this ultimate absolute defence just shows why Orochimaru was once called Sannin. The gates of carnage, arrived from hell easily chase away the opponent's power, seemingly if ridiculing their screaming. After that, nothing remains but for the person bitten back by Orochimaru to embrace great despair…
- Mechanical Light Shield Block - A mysterious chakra shining with splendour becomes a shield boasting a strong defence!! The chakra spreads out like a thin film and turns into a protective sheet, preventing all physical attacks and keeping the user safeguarded…!!
- Kamui - The target can struggle helplessly, but against this technique no defence is possible. No matter what, the space will distort and draw the target in.
- Blocking Technique Absorption Seal - All techniques are absorbed and made ineffective, regardless of the nature of the chakra.
- Formation of Ten Thousand Snakes - In front of such overwhelming numbers, a true "wall of snakes," defence and evasion mean nothing. The enemy can do nothing but become buried by the snakes. Furthermore, when the inexhaustibly crawling snakes gather, they become a defensive wall, blocking any kind of attack!!
All these techniques given by the databook as near invincible defense, yet no one has given any respect to them, even if they haven't been overpowered yet.
Also, if the Amaterasu was really as hot as the sun, the entire area, let alone the opponent and the user, would be burned to ash within a few
seconds. Its the strain of maintaining the Amaterasu that makes the eye bleed.