The problem is that is scales dramatically if not pressured (like in treaty), but you generally have to sacrifice most of those benefits to survive in supremacy.
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In age 2 you have huge gaps in your roster (no light infantry or cavalry capable of raiding), but once you get to get to age 3 you get uber versions of both.
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You don’t get cow shipments like Hausa, so either you cash them in once or make a risky long term investment. Even Big Benny has a massive long term advantage if you save it until after you research Cow Loans.
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Cards like Loyal Warriors make all your auxiliary units 15% more cost effective if you can wait until age 3 before you start making natives and mercs. Other cards like Cartridge Currency further boost your late game economy.
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Techs like Church Organs have a dramatically better payoff if you can wait until you have more Monasteries.
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There are a ton of individually mediocre cards and alliance techs that add up to a solid set of buffs once you send them all.
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You can only get one Sebastopol in age 3 and its stats are brutal at that age (and still not good in later ages).
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There aren’t a lot of good unit shipments. And those that are good cost a huge amount of influence.
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Most of your upgrades are very cost effective, but have a huge upfront cost that is hard to come up with in supremacy.
In supremacy, you just have to give up on most of those advantages since you’ll have to cash in all your cows and spend all your influence to create an overwhelming mass of units to actually win. If the game drags on the advantages start to pile up, but generally you have to claw back from a deficit if your timing push fails. Usually, the point where you should start to take off is about the time all your Monastaries run dry and safe hunts are used up, so your economy is in shambles when your units become good.
Overall, they just aren’t very fun because your are super restricted in what you can do in age 2 due to unit limitations, and in age 3, their finicky economy falls apart shortly after you start getting your strong units and cards. You basically have to do an early age 3 timing push or grind it out until the very late game.