Arrows of Harming do too much damage, with one arrow of harming II being able to kill a player in one shot without enchantments. And since magic damage isn't blocked by armor, it can kill a player wearing diamond armor in two shots. They are also too cheap, incongruently with the rest of arrows.

Are harming arrows op?

TL;DR: Arrows of Harming/Instant Damage are a great way to extremely quickly kill pretty much any non boss and are nearly impossible to defend against.

How much damage does harming 2 arrows do?

Tipped arrows of Instant Damage II inflict 12 damage upon impact. This damage overrides the arrow's damage because of how damage immunity works. Armor does not protect against potion damage, however, the Protection enchantments does.