This is likely somehow related to #45592 (or at least some findings I've made while investigating that). I'm not saying that this is a duplicate, but likely we should re-evaluate the overall situation once we get stuff related to #45592 fixed (I've already had various patches for that in testing for a long time, but they've only lead to new cans of worms which I've then been forced to look at first)
While testing auto-games with v3.0, I have noticed that the AIs always build the same military units, and there are many units they never build: artillery (from catapults to howitzers), submarines, carriers, air units (no planes nor missiles).
I know the AI can't handle fuel units and never build them. In previous versions at least they used missiles, and civ2civ3 rules allowed them to attack planes in order to give the AI some counter, but I no longer see any missiles in my games.
In previous versions, I used to see pairs of musketeers and cannons trying to attack me, but I no longer see any unit with high attack and low defense, not even archers or legions. They use some cavalry, but most of the time they just use infantry for both attack and defense. I have tried to change the class of the artillery from "Big Land" to "Land", in case those limitations were affecting, but I still did not see AI building them.
In my tests (mostly with 3.0.6 + patch #42169), the AI practically only build units with the roles DefendGood or DefendOk. Usually it chooses one unit with DefendGood and every city build the same one.
- When I edit the ruleset to remove those 2 roles from every unit, the AI hardly build any unit, and they lose cities easily. I guess it is not a bug, I just point that AI behavior depends a lot on those roles.
- It is when I remove DefendGood from every unit, and I use only DefendOk, when the AI seems to start using a good variety of units: I have seen legions, alpine troops, knights, and sometimes a cannon or howitzer. I'm testing now with my custom version of the ruleset, but it might be a good idea to adjust these "roles" to the new versions of freeciv. I did that for 2.5 and I have not readjusted them since then.
I'd appreciate some input from those who actually know the related AI code.