Well, I donīt have so much to say.
But, recently, iīve discovered that the weaknesses of the main cannons are bigger than they advantages.
Their most problem is that they do pretty low damage. DI can remedy that a little bit, and Seth Will can do that as well. But, still, Gun and Spear can do a lot more damage using a similar formation.
Besides, their extremely low defense, makes killing them pretty easy.
Princess can heal them every turn, but, even with that, meanwhile the enemy kills your mercenaries, you still be vulnerable for spreading attacks. (Spear,Gun, Instrument,...)
My conclusion: Cannons in PvP has lost a quite bit of power. How bad.