Cannon ability is bad because cannon are rare (like what... 3 or 4 event with cannons? Idk if fireworks nodes count as cannon or not), it only add a target to to cannon (it will be interesting if it was a cannon damage boost but not) and anyway, most of the time, there is not that much ennemies remaning when a squad pass through a cannon node.
But if it affect all the squads and not only the one with the girl, it's less bad than expected. Fortunatly, thoses girls also have a +10% damage for whole team and respectivly a whole target and 2 target skill. It could be worse.