"Everything is hitpoints because deep down everything is 0s and 1s on computer and that's just numbers. See that 3D model? Hitpoints! That texture over there? Also hitpoints!"
Please, that's not what I'm talking about. Granular HPs (general or on body parts) are hps and you roll for damages to hps, these systems are the most common because they work better, unlike systems where you rarely get hit (the attacker needs to roll for damages superior to your armor or something) then you roll to determine which injury you get and you only have a set of injuries with the maluses which come with it.
Granularity is good, hps are good. Body part hps that when they hit 0 you're dead or you lose the member if it's not vital or something like that are fine, with or without direct targetting, but they do not change the game much.
Now about lost of efficiency directly based on remaining hps, fundamentally it has nothing to do with body parts, in The Banner Saga your damages depends on your hps, and the game uses general hps. Now coupled with the initiative system of The Banner Saga it's ridiculous, a damaged character is worse than a dead character, but even without a retarded initiative system you must be aware of the dead man walking aspect of it when implementing such things, the induced vicious circle that when you get hit by an opponent then the next time it's harder for you to hit him and easier for him to hit you is not really a good thing, I think the worse part of it is that when you're almost dead then you're basically dead, that's not really exciting, on the contrary what's exciting is winning a fight with your party almost totally dead (knocked-out), which is also why hp systems are superior to systems without hps, I think possible injuries based on critical hits on a body part something like that (body part targetting is not even mandatory) is a better idea than directly basing efficiency on body part hps.
Sure lost of efficiency directly based on remaining hps is better with body parts than without body parts. The maluses are different depending on the damaged part, so at least in theory you may often be almost dead without being basically dead. There are different chances to hit depending on the weapon, monsters have different protections to different body parts and armors offer different protections to different body parts, body parts have different max hps. Therefore you may at least sometimes want to target different body parts, which adds some choice. That's cool but in practice the dead man walking aspect still exists at least to some extent so you can't totally exclude it from the equation.