A Dedicated Crossbowman is a massively situational class. You qualify if you have over 150 in crossbow.
It's performance is based on:
1:
The Weather. Due to rain, the class is
crippled one out of every two games.
2:
The Map and Gametype. A Crossbowman's ideal situation is a map and gametype that allows him to avoid melee, cavalry and have cover from archery the majority of the round. This avoidance allows him to actually be a crossbowman, getting off a reasonable number of shots before getting ganked.
I must say, I don't like how powerful the class is at all. Neither do I like how their heirlooms are so insane that they can oneshot armoured people while having a closed crosshair all the time.
The damage is justified by the immense loading time and crippling equipment limitations. You also exaggerate, the Arbalest does not one shot everyone and everything everytime. It's damage is variable to a great degree, as per example, I was unable to one hit kill a naked archer in the chest at 20m on three separate attempts. Yet a headshot under 10m on an Armet will kill two out of three times.