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In my opinion, all these machines (except for the catapult and the wagon) are only useful in the mid-game or on small maps.
The longer the game lasts and the higher the hero's level, the less important combat machines become.
Although, I don't really like developing machines. They require a lot of money, and in the beginning, every gold piece counts, and there are many other, equally useful skills.
Unfortunately, you are right. They cost a lot of money.
Added 2 minutes ago
What's the point? The ballista will become useless after the first Armageddon and that's it... Consider that you cast Armageddon once (while enchanting).
Perhaps, it would be worth increasing the ballista's hit points?
Added 2 minutes ago
The catapult, with its unpredictable behavior, "slightly" disappointed me, and after seeing what the earthquake does, I gave up on it. The catapult already exists, and I don't think it's worth upgrading it; it's not very useful, that's what I mean.
It makes sense to upgrade it if your hero is a machinist, or if you need to capture many cities.
Added 7 minutes ago
Why lose a dozen experience points for what? So that the catapult only does its job after 3 turns, when the earthquake does it at once, the tent revives several units, when raising the dead for the Necromancer and resurrection for everyone restores everyone, I have nothing against the ballista, especially for the Devils and Elves, which I don't play.
Yes, I agree with you. It doesn't make sense if you're not a machinist. However, in some cases, you need to upgrade machines to get a Super ability