The bombard unit does area damage. If there are three zombies (there are usually a lot more), and the bombard unit does 10 damage, then it does 30 damage with every hit. On the other hand, the other two units shoot only one zombie at a time. For them to keep up, they'd need to do around 30 damage in the time it takes the bombard unit to shoot once. The other two units need to be heavily rebalanced, if you only use the bombard unit the game is too easy. If you use the other two units at all, the game is freakishly hard.
 "Sentry Knight"){kind=icon}
 "Zombo Buster"){kind=icon}
Thanks for the report! Will look into it now.