In Ember's combat system I've made it so attacks hit a random hit location unless the attacker makes a "called shot" to a specific location.

Currently suffering damage works like this, if you take a certain amount of damage to a limb that limb is disables until it is healed (broken, muscle damage, etc), if you take a certain amount of damage to the torso or head you are dazed and lose one of your 2 actions on every turn until you're healed.  After these points there is a second damage threshold, if this threshold is met for a limb that limb is destroyed, cut off, smashed to pieces, whatever.  If it's met for the head or the torso the character is dying and has his toughness x 2 rounds to  be stabilized or die.

My question is does this make sense/work?

Am I overcomplicating things with the hit location system where every body part has a certain amount of health?

Would one "HP" pool with rules for excessive damage to individual limbs perhaps be a better less complex route?

Any advice or thoughts would be great?  I haven't seen an RPG with hit locations in a long time and I'm wondering if there's a reason for that?

That description makes sense.  Whether it works or whether you're overcomplicating things is a matter of opinion, and what effect on gameplay you desire from this mechanic. 

I did a quick Forge search for words you've used to describe the effect you're going for with your combat system, and this is what I found:

Fast
Logical
Deadly
Realistic

Fast and Deadly?  Those are things you can get by tweaking the system to your liking.  There are some elements in the mechanic you've described that might end up working against the interests of speed - specifically the multiple levels of Accuracy / Hit Location / Damage / Effect.  Of course, this is largely dependent upon how the players handle the mechanic.  If they breeze through all these steps in 25-30 seconds, that may or may not be an acceptable speed.  I imagine deadliness is just a matter of fiddling with the numbers - HP and difficulty to hit per bodypart, etc. - until you get what you like.  To some extent, Speed and Deadliness work hand in hand - raise the deadliness of combat and you'll speed up deathmatches overall, even if every iteration of describing a "single attack" takes a while.

"Realistic" and "logical" are largely matters of opinion here.  That is, what one person thinks is realistic or logical may not be to another.  If we define realism as "a high level of detail on injuries" hit locations are just one common way of getting there.  Again, just as with Deadliness, the numbers can be tweaked on your thresholds until you get what you want.

For the following, I will define realism as the notion I have of what would happen in a fight (which may be different from someone else's notion!).  Now if I understand you, the following is true:

"If a limb takes twice the damage it would take to disable it, then it is destroyed."

First, do you mean "twice the damage total" or "twice the damage in a single attack"?  IMHO, the latter would be more "realistic".  Simply because an arm is injured already does not make it a whole lot more likely that the same arm would be destroyed with another attack.

As far as the single hit point pool, I'd say that would be less "realistic" as well, and not necessarily a whole lot faster, though perhaps a bit less complex and easier to teach.  But this is all a matter of opinion.

All of this said, I'm not a big fan of hit locations or HP.  The former because I don't demand a lot of system-coded detail of description.  The latter because I think numbers make more sense when used in other ways.  While I'm probably not your target audience, I've tried to provide useful feedback.

-JT