What makes a justu forbidden?

[Jin Hayami]

Overall dangerous.

 

[iSpeak the Truth]

This: If a jutsu goes against the natural order of the world, ex.... Edo Tensei, Izanagi, Jigoku, Immortality Jutsu.

If a jutsu poses an extreme amount of risk to the users life, ex.....Chiyo's Puppet awakening jutsu, Tajuu Kage Bunshin.

 

[Strata Uchiha]

If it puts extreme stress on the user's body such as Guy's Afternoon Tiger. It's a double-edged sword. He puts a hurting on his foe, but afterward he is extremely incapacitated.