The crossbow is relatively inaccurate and requires a long reload time. Beyond that, if 80% of players are using crossbows crossbows are amazingly balanced seemingly as every server i go on has a majority of kills done by melee (i'd say at least 70%) unless it's the start of as siege when attackers are rushing into a defensive wall.
Ranged characters have to adjust for drop, flight speed, lateral movement. The crossbow itself is around 2 pounds, the bolts are also 2 leaving you with 4 to 5 extra pounds to carry about, a significant amount if one does not have high athletics (speed is king).
While i've been killed by bows from time to time i've never felt they were broken because of the general level of skill by the players who kill me. When a guy fires from a huge distance away from me while i'm running I swear under my breath a little, look over, and have to admit- hell yeah he earned that kill.
Of course i'm of the strange opinion that the fact that people can kill me in a variety of ways, and that the game caters to a variety of playstyles as a positive. Nothing wrong with competition or variety.