Evil biomes in general are more RNG-dependent than other biomes if you are looking for something specific.
The first thing to check an evil biome for is death, "dead" evil biomes have no living flora, and won't regrow flora with time, meaning wood becomes a finite resource here, and you'll require to harvest the seeds of any wild plant you can for farming in order to maintain a supply of sliver barbs for example.
Then there's reanimation, this is very simply if the biome has undead or not, if you can find native undeads on your embarking biome, it's reanimating, and therefore the !!FUN!! kind, else it simply has the evil-aligned fauna according to the dryness/wetness, temperature and shrubbery of the biome, it's usually on the non-reanimating ones that you can find creatures such as harpies, beak dogs, foul blendecs, etc...
Finally there's density, this means how dense is the "evil" in the region, like say, how dense are the growths of wormy tendrils and/or staring eyeballs, or if it has them at all. This means that sometimes an evil biome can have giant undead creatures roaming around daily, yet regular grasses and flowers grow on the dirt of the region, with no tendrils or eyeballs visible at all on the ground.
With only these 3 variables at play, the variety of evil biomes really becomes prevalent on every world generated, especially since traditionally, "natural" evil regions are quite rare on the first place, thus any specific flora, fauna or features that you're looking for may be hard to come by when you actually find them in the first place.
If you're looking to generate a world with a lot more good/evil regions, on the advanced generation parameters, go to the "evil/good square count" settings, and simply increase those, they tend to create more "spots" of evil/good where you may be able to find what you're looking for.