In Americas the Culverins are battle-winning on offense. Defensively they are not as useful due to the incredible running speed of native units, and they will be on top of you after only a few shots.
In any case, artillery can give you a huge advantage, although you may not be able to see it. They may not kill 50% of the enemy army, but even if your artillery only kills 10% of one unit, and even ignoring morale penalty: that unit is that much weaker, that much quicker to rout, and that much quicker to start a chain reaction.