Well, civs with heavy units, at least, so they’re worth converting.
Heresy is a 1k gold investment. If you’re forcing them to do that then thats 22 crossbows or 13.3 knights they cannot train or that is a bunch of age up gold flushed away on conversion immunity.
As much as we say elephant civs would be unstoppable without this tech dont forget elephants need lots of gold too whicb is why the civ that halves the cost of heresy definitely need to lose it for elephant sake
Heresy really shines in Monk wars. It is one reason why I do not have as high a view of the Bengali Monks as other players, while I do not deny the value that their extra armor provides.
That said, I do not really see the issue.
Mongol mangudai. Simple as that. Mangudai are one of the hardest units in the game to counter, especially for civs like slavs, who have better monks than skirmishers.
Mangudai have the same problem other wincon castle units have and thats the fact a castle is more expensive than a monastery or going stables with lancers instead. The problem for slavs countering wiry monks is the beefed up light cav that chop through them