Do you use them at all? They seem unpopular in single and multiplayer, like ''why use them if you have organ/helstorm''. Sadly there is no lord who would have them in starting army.
Maybe they are like european cannon from Shogun 2 (not FOTS where arty was awesome) which were in fact very weak.
Cannons were fun to use in BOTET (Warhammer mod for Medieval 2), where it was really fun to watch how cannonballs plow through enemy infantry. Using them for sniping large targets is kinda anticlimactic imho.