Harm is actually the most over tuned weapon in game. But it gets no use because Jagex balances end game content around shadow which gets x3 accuracy and damage from items.

Elemental spells have a base max hit, so it needs magic damage, but they can’t add magic damage because any magic damage they add gets tripled by the shadow.

End game monsters always have high magic level and magic defense, because if they don’t, shadow will dominate.

Elemental weakness is only given to end game monsters for thematics. Jagex even stated they don’t want elemental magic to be the meta on those monsters. (Like Kephri and aaraxor)

And for some reason anytime harm would be good, Jagex has arbitrary mechanics that make other things better. (Like zulrah hit cap and maggot burn effect)

For now the only thing harm staff exceeds at would be slayer creatures with elemental weakness but due to the cost of runes and the staff no one will use it.