cannons are difficult to use with more complex mechanisms. 
the cannons are incredibly heavy, which isn't helped by the fact that they only connect to one block at a time.

i tried swapping a cannon out for the flamethrower on a working tank turret i was experimenting with, and it seems to be simply impossible to make it work well. it sort of works, but it bobs up and down and the cannon has a tendency to just fall right off.
by comparison, the flamethrower barely seems to weigh anything.

i would love to see cannons either
A) get buffed (they are far too weak to justify the ridiculous weight and recoil force) excellent range in the update and the recoil is now so absurd that it provides comedic value.

or

B) have a connection point on each of the 4 "long" sides, which would make them infinitely more versatile  when thinking outside the box.


and in case you don't believe me on the recoil being crazy, i made a little demo on Youtube.

www.youtube.com/watch?v=dKAJGiqLTI0

as you can see, the cannonballs barely make it over the first wall, while the recoil is enough to send the 4 entire cannons almost clear over the whole fort.


edit: just saw the new version and updated.