Just because a power exists doesn't mean that it's always a great idea to use it. In fact, some are so dangerous that they're straight-up forbidden - or at least, their users know to be extremely judicious about when they resort to them.
Some forbidden anime powers are banned for their hugely detrimental impact on the user. The Eight Inner Gates, a taijutsu technique from Naruto, almost inevitably results in the user's demise. Others have the potential to create mass casualties. The Athena Exclamation from Saint Seiya is the rough equivalent of the Big Bang.
Do you think that these moves are banned for good reason?