I've never been entirely satisfied with the Critical Injury mechanic. It's an old mechanic, based on 1st Ed Warhammer Fantasy Roleplay, and it was innovative at the time (1983-ish), but it hasn't at all weathered the intervening years. The main foil of the RAW combat mechanics is weapon Pen values.
That being said, I've never been entirely satisfied with the interactive mechanics of AP, Pen, TB, and the various Damages (Falling, the myriad weapons, Toxic, etc). I've drastically changed the way these mechanics work, and I may someday sit down to share it with the forums, but for now I'll stick to Critical Injuries.
Many people on the forums have lamented that a Stunning hit to the Head leads to Arms and Legs being completely ripped off, and it's a case of "Well, that escalated quickly." Here's how I've changed the Critical Injury mechanic:
First, all types of Damage have a modifier, and most times this can be found listed in a weapon's profile or is associated with the SB of a hit. A Stub Automatic does d10+3 Damage (Type I), Pen 0; the Damage modifier for this weapon is +3 (the additional Damage added to the d10 roll). For Melee weapons, the Damage modifier is equal to the SB of the attacker, rather than the additional Damage added to d10 rolls. Unnatural Characteristic (Strength, X2) would raise a Damage modifier from 3 to 6 (we're all pretty familiar with this mechanic).
Now, whenever a PC first takes Damage that would decrease their Wounds score to a total of -1 to -10, the attack is considered as taking their score to 0 (zero), and then the Damage modifier is added to 1d5. Gleeb has 4 Wounds remaining and is hit with a .54 Tranter Handcannon (d10+5 Damage), taking 7 Wounds (after reduction for Armour and TB). Normally, Gleeb would have a -3 Critical Injury, but the next several hits could result in minor Damage and Critical Injuries of -4, -5, -7. With my House Rule it works as follows: instead of dropping to -3, Gleeb's Wounds are reduced by d5 plus the Damage modifier (in this case, 5), for a total Critical Injury of -6 to -10. Personally, I feel this represents the hitting power of a .54 Tranter rather well. If Gleeb had been hit with the Stub Auto, the Crit would range from -4 to -8 (1d5+3), and if he were hit by a Great Sword wielded by a creature with Unnatural Strength 4(x2) the Crit would range from -9 to -13, or 1d5+8 (which is well beyond the Critical Injury Tables' ability to quantify). If a weapon happens to not have a Damage modifier (like a Needle Rifle, Damage d10, Pen 0, Toxic) the Critical Injury is simply 1d5.
Now, in the case of Toxic, Falling or from being on fire, the Damage caused by the die roll is interpreted RAW, so it will reduce Wounds to a lower number or to a Critical Injury level as appropriate (IE: Gleeb has 1 Wound remaining, but fails a Toughness Test vs Toxic and takes 6 Damage, putting him at -5
True Grit halves the total Critical Injury (the total negative number generated), rounded down.
If the PC has a Critical Injury (say -4) and they again take Damage after reduction by Armour or TB, you repeat the process, and add the resulting number to the current negative score. Needless to say, Critical Injuries kill PCs very quickly. Those with True Grit die half as fast (@ 1:2).
Now, I will again mention I have changed many of the AP, Pen, and TB mechanics (the PCs start with a significantly higher Wounds score, @ double, and TB doesn't reduce incoming Damage from weapons), but I have noticed less "power creep" in my game; the Players are content to have their PC shot at with Las-carbines and Auto Pistols, or to be hit with Truncheons and Swords, because the Damage modifiers are relatively low, and they don't want to escalate their opponents' hitting power commensurately, so they tend to keep their own weapon selections fairly tame (if sometimes Exotic). That and the fact most of their investigations-turned-firefight/melee are in public areas and wearing Armour is too conspicuous. Sure, the PCs know the bloop is going to hit the fan right in the middle of the downhive cafe', but why advertise you're looking for a brawl from the moment you wlak in the door by wearing Guard Flak Armour. Besides, three average hits with a Las-carbine can put someone into Critical Injury status (I view Wounds like "combat luck", but my Players translate the Damage as actual injuries, which is fine, and when they are close to Critcals they worry, and that's fine too, as they are immersed). They know if they're getting that low on Wounds it's time to mitigate Damage by spending Fate Points. With this mechanic, Critical Injuries are exactly that, critical, and Fate Points can also be burned at a more rapid pace as a result. Well, rapid as in one per every other combat encounter However, with the initiation of this Critical Injury mechanic, I have decided to be far less stingy when awarding Fate Points upon the completion of significant goals (still capping the max at 5). I don't pull punches anymore, however. The dice fall where the dice fall.
This mechanic has been back-designed for RAW. It may not be to your liking, too deadly. It's a base to "fiddle" with, to get you started thinking.