Pretty sure (don’t quote me on this) that the final plan for difficulty curve is a large population of regular zeds that slowly mutate into more powerful forms (specials like shockers, spitters, brutes and hulks) as time passes. So the first city you raid will be a relatively simple matter, but later in the game attacking a city will be more complicated, as you’ll have to contend with foes that can attack from a range, do more damage, etc.