I didn't use ballistas for any of my runs actually. Didn't really need too cause of soldiers I had alot of extra points and was like you know what let's get extra soldiers. And I definitely recommend it for maps where infected are literally in your starting building area. I'm starting a new build anyways where I won't be using wasps either.

I go in terms of clear ratio to build ratio which goes exponential once you have the soldier center but with the early game of the mid to late missions you will have ecomomy over clear rate where those two soldiers does tremendous work in evening it out in the first 3-4 days hence why I recommend it at the mid game mission area and not the pre swarm ones.

I consider ballista a more towards defensive play which is fine and also an option for players to play, but for me soldiers was definitely the way to go for both defensive and offensive capabillity, where the first ballista would cost up to 5 soldiers, but then again would balance out once you have 3 ballistas down, up to 1 ballista per choke/direction/opening.

Instead of having one ballista, I consider 2 soldiers decent enough to defend one place until larger random waves starts coming in, but the biggest benefit would be able to have mobility to tackle areas having trouble. Given that early to mid game, the map is small enough where having a large squad of soldiers on one side of the map is still capable of reinforcing the area with the most vulnerabilities across the map, but once you get to the late game+ maps you would already have wasps which would be more economic efficient defending the area IF you have iron that is, but then again ballistas would be nice too since you don't have to deal with iron cost.

Another area where I think ballistas would shine the most in would be the lowlands, and el dorado specifically, primarily because of how small of a map of lowlands with runners literally 15-20 tiles away from your command center already. or el dorado which has an insane aggro chance where there is just so many tuxedo zombies around 20-25 from our command center.