Unlike Luffy, Zoro is capable to fight at his full capacity cause all his Swordmanship Can be done in base, his attack aren't based on transformation.
G4 Boundman sacrifice Speed, G4 snakeman strenght and at the same Time mess with his CoA reserve.
Its why Oda has to give him G5, G4 limit luffy alot, and because G4 already mess with his reserve of haki,its harder for Luffy to keep using adv CoA and Adv CoC together like KoH Zoro do.
With G5 Luffy has Speed, strenght Can control his Haki use more efficiently and use adv CoA and Ad CoC without trouble or running out of haki.