Can someone please explain why some build up photon art require multiple charges to use? Why does the cannon (which is arguably the most powerful) only need 1 build up charge but the sword requires 3 charges to use once? This seems kinda backwards to me.

It might be based on how frequently the weapon is used, the launcher is a secondary weapon for me as a Ranger, and I go for the rifle most of the time. I end up using the Build Up PA mainly on bosses to take them down faster and sometimes I need 3 attacks on the same boss or to drop two smaller bosses quickly with single attacks each.

I think having the larger build up on a primary weapon makes it so you can't spam a very powerful attack, making it so you still need to learn other class abilities. By default it takes doing a lot of damage to even get one charge on a Build Up PA ready.