Evil biomes are heavily procedurally generated. An embark might have just regular rain and snow as weather. It might rain harmless elf blood. It might rain brown sludge that causes dwarves to be mildly dizzy with no long term effects. It might rain slime that instantly causes everything to dissolve. Totally random.
Evil biomes may or may not have undead. Some evil regions are swarming with infinite undead, where new ones appear the moment the prior ones are dispatched. This makes small embarks on these biomes exceptionally difficult. The undead are relentless. They never sleep, never run away, and there's always another one. You will be under constant attack from day 1 of your embark. Other times, evil biomes have regular living creatures and no undead walking on to the embark from the edge of the map. You might get harpies instead of red pandas, but harpies at least die and stay dead.
Some evil biomes can have spontaneously regenerating undead, where non-mangled corpses will spontaneously get back up. You may have to kill the same cave crocodile 15 times before it finally stays down for good.
Sometimes you get clouds. Clouds are not raining elf blood or green sludge. Clouds are different. Clouds can be harmless, or they can be instantly lethal, or they can turn living creatures into husks opposed to all life, and the dust in the cloud spreads and infects everything.
And to add to this, evil biomes can have different effects at different z-levels! You might get a safe biome at ground level where the undead do not reanimate. But go up just 1 z level and now everything is reanimating and there are husks all over the place.
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All of these variables are independent from each other. There are so many factors that embarking on an evil biome is often luck.
Note that evil clouds can take a while to show up. Evil clouds might appear only 2-5 times a year, so you might think everything is okay at first, then in autumn a cloud rolls on over your trade depot.
The above findings are based on my personal experience from having played DF for more hours than I'd care to admit, going even back to the 2nd version before spontaneously reanimating undead were even a thing.
Funny thing is, I think I'm the person who suggested spontaneously reanimating undead in the first place. I think Toady One read my suggestion thread and actually did it.
On some embarks I regret my suggestion! Undead ogres who will just not stay dead are no joke!