I have no idea why its so hyped OP TBH.
Neji's part 1 kaiten was already overwhelmed by a kunai strike from a Kyuubi chakra enhanced Naruto (not even 1 v1 tail of power).
Its not a strong defence.
Key words:
Part I Neji.... a Genin. Additionally, Neji did not make the full rotation as I recall he actually crosses Kunai with Naruto (not exactly going to work during rotation).
OT:
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^^^^ This is the face I make when you bully the Hyuga Clan (I do not seriously mean stop posting... that's immature. I simply refer to the comical actions she takes

).
The strength in rotation comes from its simplicity. Instead of being enhanced with an element (which presents an elemental weakness), Rotation is pure chakra. By rotating, it allows someone to create a shield that will block all attacks
during the duration of the spin..
Naturally, the Jutsu is almost useless when the user stops spinning. However when it isn't it is an absolute shield. Firstly look at this The Mokuton Spikes are blowing into the ground and have enough piercing power to crack through the ground and the Earth Shield by several feet
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. It's even significantly passing through people and armor
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. Thirdly, upon inpact the Spikes Seemingly grow and disperse making them blow around a body
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However, observe as they hit the Kaiten... they simply bounce away with no effect
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. As long as the Hyuga Maintains a spin, the defense holds against even piercing strikes. More noticeably, it only took two Hyuga using rotation to repel the Juubi's gigantic hand (which is probably the size of a Bijuu)
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. The technique itself would have protected either Neji or Hiashi; however, they used it in conjunction to prevent it from injuring the people behind him (Naruto, Hinata, other Shinobi).
The Jutsu fits in with the Gentle Fist style in that it is not designed to be spammed and repeatedly used, but instead be a just guard (Soul Caliber reference) that lasts long enough to block a long range attack...and can be immediately followed up by rushing forward to close the gap against the enemy.
As we saw, the Juubi had an unnaturally long spanning attack, and as the time continued, the Spinning stopped, and had to be re-started. So even then, while not being used for its most practical intent, the Kaiten was still holding strong as a defensive shield.