Indeed, I never figured out why it's so weak in comparison. My Barrage II promoted longbowman oneshotted a full hp spearman, while it would take a stength 18 city
atleast 3 turns to do the same, probably longer
In addition, what are the real benefits from Garrisoning units? From my experience, you keep the archer/longbow/crossbow garrisoned, then use the city ranged attack, then select your garrisoned unit and use its ranged attack aswell. Is "City Garrison" a fancy word for letting a unit stay parked on the city tile or what?