Yeah, a terrible synergy. Cheaper/stronger castles which are difficult to take down with trebs sucks. Especially if you are playing as a civ without BBCs, like vikings, japanese, sicilians, or even mayans, it sucks having to deal with frank castle spam with BBCs.
Franks are rather strong in the late game, so I don’t understand why the berry bonus was given to them. This could be removed instead.
But Vikings have Champions with bonus damage against Cavalry and more HP against the Franks Paladin and Arbalest against Axemen/HC. I would be rather confident with Vikings against Franks, but Vikings are imo overall the strongest civ, so maybe I am biased.
Japanese have faster Trebs and better Halberdier. Samurai maybe against the Axemen, but HCs likely become a big problem. But Japanese overall kinda suck if they make no good use of their combination of bonus. They have to convert their cheap economic building bonus and/or fishing binus into Infantry spam in Feudal and Castle, otherwise they kinda suck against many civs imo.
Mayans and other american civs are supposed to be weak against gunpowder in imperial age.
Okay, wait. I’m specifically talking about BBCs. Don’t generalize this to “other aspects”. You want to give strong infantry like with bulgarians, or gunpowder (except BBC), go ahead.
An interessting aspect of Franks are their castles, and BBCs contribute to that. If there is a problem with Franks gunpowder than it is HC, because it makes their unique unit and therefore also their castles less important, what does not really fit their idea, and makes them maybe too strong against Infantry. But now it is maybe a little bit too late to remove HC, they could have done it in 2000. The devs should not get into the habit of removing units from established civs.
Remove the berry bonus, and see what the result is. The devs seem to agree that the berry bonus is the aspect to change, but they shy away from removing it entirely.