The Crossbow is a Sniper Rifle that is supposed to do gargantuan damage to the head.
Due to a programming error (in a similar vein to Shields that lack reflective capabilities sharing the Buckler's unparalleled accuracy even when they're not supposed to), the extra headshot damage stat on Sniper Rifles does not stack with headshot damage. This means that the Crossbow's headshots actually do less damage than a standard shot (and most noticeably critical hits) from the AWP, while also only doing slightly more damage than headshots from lower leveled Sniper Rifles (excluding the Dragunov due to its higher damage). To make matters worse, each shot requires a reload.
Although the Crossbow has higher accuracy and reserve ammo than most Sniper Rifles, it has a far lower DPS and very slow reload, thus making it one of the most overshadowed and unforgiving weapons in the game.