The last time Luffy tried gear stacking for a single attack in Thriller Bark, he ended up in a comatose state. He improved his G2 control though, so we might not know how much he can handle gear stacking now, but saying G4 has more drawbacks is completely baseless.
Besides Gear stacking's main advantage is to take advantage of g2's speed to land a devastating G3 level blow, maybe with additional G2 level increase effect, but in the long run, G4 seems like the bigger PU, especially when it grants you better defense, stronger attack, grants you ability to fly and with different modes(tankman for even greater defense, boundman for greater power and maneuvrability, Snakeman for greater speed).
Overall then, it makes much more sense that Luffy abandoned Gear Stacking for G4. G4 is much better