Problem to me is that harm circumvents one of the base mechanics of D&D, HP.
Everyone wants lots of HPs because they give a sense of longevity to characters and really distinguish high from low level.
Save v death spells are feared because they potentially can get around this, but there are defensive options such as boosting saves, Deathward etc...
Harm is a touch spell that is relatively easily delivered leaving the victim very fragile. Touch is very effective vs large+ enemies. Defenses v Harm are fewer and less effective, especially if the victim is outnumbered due to the turn based actions system D&D uses.
The classic Dragon in his lair (unable to fly) gets a lot more easier with this spell. Note easier not easy

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I believe every attack should have a defence and Harm does not.
I am not considering the potential damage output of Harm because I rate it closely with the save or die spells which potentially do a similar amount.
I have been playing with harm as is but am considering:
1) Making "Negative energy protection" an effective defence. It is lower level but not a certain counter due to the opposed roll. Freely available by level but not 100%, only problem here is that it may make NEP a "must-have" spell. That i can do without.
2) A save, likely will, like the inflict spells. Harm would possibly need to be reduced to 5th level with slay living being moved to 6th...
3) Cap damage to 1/2 HP. The spell has diminishing returns beyond the first hit but still inflicts great damage.
4) NEP and 1/2 HP cap? Other combos
GAAAAAAAAAAAAAAAAHHHHH