Like kyo said, "less useful geetar"
The way I see it, that's all the cannon main is though that's comparing 100+ cannon to guitar class. Cannon has the benefit of + hit where guitars can only hit 2 targets at most and iirc the damage amount is the same or fairly close.
The fact that you get Devastation Shell at 100+ evens the playing field in comparison because quite frankly, Ravaging Melody is hax pre-100, though that's not saying it isn't hax past 100 because it still is but at least now Cannon's also have a similar weapon.
Cannon's and Guitar's could theoretically use the same build.
@Dook, everyone's power relies on their mercs. Cannon's and Guitar's just make the job of your sidekicks that much easier.