On the flipside, I find harpies and beakdogs to be far more dangerous than undead. I am unsure why, but I am almost always able to keep the undead out; they seem to prefer to stick to the edges of my maps and are only occasionally a problem. I have allot of problems when my map has harpies; sure, they only come in twos and threes, but as soon as I kill one pack another spawns. then another. soon they overwhelm the first seven with wave after wave after wave that zero in on my wagon and win the battle of attrition by sheer weight of numbers. eventually they get lucky, and a dwarf is injured; then another. then the woodcutter\proficient axedwarf slips up and they rip out his throat; there goes the axe. both miners have yellow arm wounds. The planters are running for their lives and finally the caravan shows up; but it's only a momentary respite. soon they (and the guards they bring) will be gone, and the assaults will continue until no dwarf is standing.
Of course, ever since I stopped a pack of tuna with a single constructed wall I've viewed such impenetrable defenses as being a bit cheap and steer clear of them, so that may have some influence on my poor luck with the endless waves of harpies and beakdogs. Harpies are worse for their speed, but a large pack of beakdogs spells serious trouble if they catch sight of your starting seven.
Werewolves are kind of pathetic really, I had an untrained weaver snap ones neck in his sleep once. Flipside though, werewolves mean foul blendecs, and those bastards are mean.
Biggest problem with evil is you may or may not have a dead map. dead map means no live trees, so once you've cut them down thats it. No plants for gathering either, so if you want to do surface farming, better hope you can get the humans to bring seeds.
Fun maps overall though; if you're getting bored they provide allot more action. also, if you have a map with lots of skeletons: endless supply of bones.