Errrr hmmm. And how many foes have you had that got down to 4 h.p. and then managed to beat the party? Properly timed, as in for example 'I hold my action until the cleric casts harm.', harm is an instant kill no save spell.
But broken? Hmmm, well.
1) Considering the average h.p. of a CR 13+ creature, it vastly exceeds the average damage per level a 6th level clerical spell is supposed to be able to do, even with the touch restriction.
2) It has no saving throw.
3) There are considerable more complaints against it wrecking compaigns than Destruction, which is 7th level and therefore presumably more potent. There are vastly more complaints against it than Implosion, which is 9th level.
4) The core spell with the most similar 'die no save' mechanics is 'Power Word Kill', which compared to harm has an upper limit of 100 h.p., is 9th level, and is arcane.
Therefore the whole debate that harm is not broken seems absolutely frivilous to me. The only saving grace of harm as written is that it is a touch spell. Even so, it needs to be rewritten to either have a save (like a weaker form of Destruction), or else have a h.p. cap (like weaker form of Power Word Kill), because it is otherwise a monolithic anomoly in how D&D handles spells and such anomolies are best reserved for 9th level spells if they are allowed at all.