They have a low HP to AP cost, long recharge and move slowly. Their main weakness however is that good early-game units counter them- they die in one hit to the Claw Unit with no chance to counterattack, while Utan and Donkey Slug can attack them outside their counter range.
It also helps that they have one of the fastest productions speeds of all units and are pathetically cheap to produce, which allows one to keep pumping out scientist zombies until their combined Counter-Attack is able to destroy boss-type units like Donald Morden in one fell swoop. The scientist zombies have lengthy invincibility frames on knockback, allowing them to survive where other infantry normally wouldn't.See Game-Breaker below or here to understand why. Parachute Trucks in Wifi battles, until the 1.30 update where they got a much-needed Nerf.
