It's meant to make up for a lack of main class multipliers that affect gunslash damage.
One thing I've noticed, and saw mentioned on the s-wiki though, is that it's balanced with average stance in mind for Br, so Br/Lu has the third highest damage potential with gunslash if you don't care about utility, because of weak stance, only beat out by Hu with max war brave, which is less likely to be relevant in a boss fight than weak stance, and Fi using wise stance. Fi sounds good on paper, but because the multiplier is so low by comparison you have have to rely on basically having everything Fi has in its arsenal except maybe chase active. That's not necessarily bad, since Lu just makes a good sub for Fi in general, but if you purely want to use gunslash it's not as convenient.
Hu is probably the best general purpose main class for using gunslash, since you get Hu's suvivability and less situational multipliers to do more damage than Fi/Lu. Fi/Lu only beats out Hu/Lu for gunslash useage with wise stance while using limit break and tech arts. Again though, this is only in the context of gunslash, and even then it's not like the weapon is terrible on Fi/Lu.
Te/Lu has potential of giving Te another bossing/solo option that doesn't rely on techs with gunslash, but I haven't tried it that much.
The rest of the classes it feels like Lu isn't a good subclass for in general, or the gunslash multipliers aren't notable enough to make them worth using specifically for that, though they do still have the option I guess. Su in particular gets shafted here because of the really low pet multiplier, AND a relatively low gunslash multiplier.
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