In Brood Wars, the ultralisk gets two very important upgrades from the ultralisk cavern. First the speed upgrade, will make the ultralisk as fast as a zergling. Second the armor upgrade can give the ultralisk a maximum armor of 6. Combined with darkswarm, the ultralisk is near invincible, especially with 400 HP.
Unfortunately, the ultralisk takes 6 supply, which is almost the amount that an overlord can provide, which can cause you to have fewer units. It's also very large, making it very uneffective at small choke points. Also choke points that use structures as supply depots to block off units, will cause the ultralisk to become very stupid, as they walk around trying to find a way in. The ultralisk also requires a lot of time before it becomes fully effective, so two evolution chambers is required.
Now the ultralisk has its good side too. Terran infantry become very ineffective against an ultralisk, because of the high armor, and darkswarm. You should always use darkwarm with an ultralisk, kinda like you should always use cloaking with wraiths. There aren't too many units that can stop an ultralisk, especially with darkswarm.
An ultralisk also makes a great defensive unit, only because it has high armor and HP. Therefore your opponents units will target the ultralisk letting your other units do damage. Zerglings usually die before they get to their target, but with ultralisks, the computer AI will make your opponent's units attack the ultralisks, letting your zerglings do the damage. Most spells as plague, irradiate, yamato gun, and psi storm require several castings to kill an ultralisk. Zealots, tanks, archons, are an ideal unit to kill ultralisks.
