This is another huge problem with the idea that gunpowder units should be in tomes, thanks for pointing it out! If they're in tomes, that means they're higher tier than the default archers and crossbowmen, so anyone who wants to go heavy on ranged units would be a fool for not taking them. Whereas, if they're a single culture's ranged units, you can simply pick other cultures. And not tying them to tomes also means that the people who never want to see gunpowder units won't end up losing out on a major upgrade to a ranged faction's playstyle. It makes skipping that DLC safer for them.
This works both ways. If gunpowder units are cultural, it means Tome based ranged units are strictly stronger. If any of those tome unit uses bows, a bow is now stronger than gunpowder. If none of them use bows, the game is restricting itself from what kind of units can be in tomes.
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