First off -- if you're describing a critical hit as opening the victim from shoulder to groin, you're probably not doing it right.
If that hit left the person alive with positive hit points, then he's functioning at no penalty -- so it's a thin slash that draws blood, will need to be looked at later, but isn't putting him down for the count.
If that hit left him in the early negatives (-2, -3), then he's down and dying but nowhere near dead, which means that it's a bloody wound to the shoulder or a glancing blow to the head. With decent medical care, without even magic, he'll be fine.
If that hit killed him (-10 to -19 or so), then it's a cleaving blow that did about what you describe.
If that hit took him to -20 or greater, than maybe it bisected the poor guy or sent guts flying off in all different directions, making raising difficult because of lost body parts.
If someone has 90 hit points and you do a critical hit that does 30 points damage, you've grazed their neck with a slash that could have very easily killed them outright. You've left them flustered at being so nearly killed, you've thrown off their balance and made them more cautious, possibly opening them to later, more deadly strikes (as evidenced by the fact that they have fewer hit points now, and ANOTHER 30-point shot will hurt them MORE than the first one did).
All that aside, it's fine to House Rule crits on undead or constructs for specific purposes.
Examples:
I made a special weapon that was essentially a dagger made to work the tools that create golems. It had the magical ability to bypass golem DR, did extra damage against them (construct-bane), and could deliver critical hits to any construct, as its blade sought the special edges and vital spots that ordinary heroes would never find.
I liked the flavor text about Vampires being vulnerable to stakes and decapitation, so I allowed vorpal weapons to score critical hits on them, and I made stakes do 1d4 piercing, 20/x2. On a crit, the vamp had to make a Fort save DC 15 or get dusted. If the vamp made its save, it was affected as though by a normal critical hit, as the stake very nearly turned it to dust and certainly messed up its undeadliness. It was in all other ways immune to crits, since those were its magical weaknesses.
In d20 Modern, there's a sample adventure with zombies. The zombies can only be killed by having their heads cut off (or something like that, I can't entirely remember offhand and am too lazy to look it up). Unless the head is cut off, they come back together after a few hours and rise again. The guide essentially says, "Decapitation can be performed on a zombie at 0 hit points or less, but occurs automatically on a critical hit. Zombies are ordinarily immune to critical hits, but if a player who knows the zombie's weakness (by having been told about it) rolls a hit that would normally have been a critical, the zombie is decapitated and instantly killed." A person who doesn't know about the zombie's weaknesses can't crit them, but the special knowledge and plot-specific info makes it possible in this case.
So if you feel like doing that, more power to you. But don't make it an always-on thing. The CR for these creatures assumes that they aren't getting critted terribly often. Most of the guys who don't get critted don't have Con scores -- they don't have THAT many hit points, so letting someone do double or triple normal damage on a crit is gonna whittle them down a lot faster than usual.
