Pre-patch Nanbans made Christianity a huge draw for Western Clans, although they're still powerful and save a lot on naval upkeep post-patch it seems a little unrealistic that ships can be struck 4-5 times with direct cannonballs and NOT sink. One pirate raid I repelled using Nanban ships saw one of the trade ships lose its main mast, get hit by half a dozen cannonballs, lose over half of its crew and still be one of the last ships to rout!

I suspect the programmers changed the way cannonballs work in Shogun 2 because of the big March patch accomodating the new expansion, but they did not document it.