Most of the deceased don't get buried in the earth in New Orleans- bodies are in above-ground crypts (think of the famous St Louis Cemetery)- I think it has to do with the high water table?? So if they were embalmed, bodies probably wouldn't rot away quickly. Of course, the idea that immortal voodoo queens, minotaurs, and witches running around is OK, but zombies couldn't happen because of the rotten flesh thing is funny in itself... We suspend our disbelief of some things, but not others.
Only a few, wealthy folks get the vault treatment. Everyone else goes into the dirt. And if you're in an above ground vault, you get baked into powder in about a year. It's complicated, neat and sort of creepy.
How it works if your family or organization or whatever has an above ground vault is that after the funeral, someone goes in and takes the body out of the casket. They open the oldest slot in the vault. What's left of the remains of the previous occupant are put down in the bottom of the vault. The new occupant is put directly into an empty slot in the vault. The casket is broken up and thrown away. 366 (this is mandated by law) days or more later, when there's a new burial and that body is the oldest occupant, they come and put the body underneath. The heat in those vaults is pretty high and it's sort of a low level cremation, so in a year, there's almost nothing left.
Go on a tour of St. Louis #1 (Ignore anything about corpses rising up out of the ground. Seriously. There are plenty of people buried IN the ground in NO. Always have been.) the guide will explain it and answer questions to your heart's content.
I'm not entirely sure that truly poor people get embalmed, they may. I know indigent, unclaimed bodies get cremated now, but if the family claims the body, not sure. Either way, even with embalming, unless the cemetery requires vaults, the weight of the earth above a casket crushes it pretty rapidly, soil gets in and hey presto, doesn't take long for the damp soil to eat away at everything.
(Many cemeteries in Louisiana don't require vaults, btw. I have friends who are always shocked by this, and offer dire warnings about how nasty human bodies are when they decompose and have germs and nasties that other animals don't, but uhm. Well. None have offered me much in the way of documentation about this, and we do it all the time in Louisiana. I've never read of a disease outbreak due to corpses in a cemetery, so, if anyone can offer scientific evidence, I'll take it, but until then, I refuse to get upset about this.)
And yeah, I've seen pictures and read reports of excavations for below ground burials in Louisiana. Unless it was in a vault, they just don't find much. Bones. Bits of metal objects. (Just FYI - there was a really, really great excavation several years ago where a family vault - just a low, above ground brick container - was excavated. They found several metal caskets from the 19th century inside and were able to do some really, really neat work from it - the clothes were extremely well preserved and the bodies were in a pretty good state of preservation as well. So it can happen in Louisiana. . .if your family has money!)
ETA: Just remembered, I've heard folks talk about doing preservation work in old cemeteries. When the families used metal coffins or good thick virgin cypress lumber and buried shallow (3 to 4 feet or so down), those coffins survive a really long time. One man was able to map out almost all the old burials in a historic cemetery from the early 19th century by using a probe. When he felt something he stopped. He said they had buried really shallow, and apparently used really good wood or metal, so the caskets were mostly intact! Verra kewl.