It's not only about hitting hard, it's about having a crap load of options, that ends into something 200% OP. Two simple builds, level 35 (which means that you can try them on your stf):
Strength:21-
Agility:21-
One Handed:82-
Crossbow:151-
Weapon Master:7-
Athletics:7-
Iron Flesh:7-
Shield:4-
Power Strike:7-
On this one, you can put 7 shield and 4 IF instead, it's up to you. Note that you can remove IF for more agi/str and get more PS or athl/wm. Keep in mind this is a level 35 build, and the level 37 counterparts are broken.
You can also abuse of the 0 slot bolts, and be a ranged player while being extremely potent with a melee weapon.
Attribute points:0
Strength:24-
Agility:21-
Two Handed:140-
Crossbow:103-
Weapon Master:7-
Athletics:7-
Iron Flesh:4-
Power Strike:8-
Another level 35 build. Over powered with loomed items, considering you can be really accurate in the range of 20/30 meters, and kill someone in 2 melee hits, while not being penalized by wearing an armor. As I said earlier, just try to guess how op it is on level 37.
The problem is xbows are a jack of all trades, they are point and click, don't require PD, nor skill from the user. They are accurate at 150wpf, and they are quite nice already around 100/120 wpf in you are close of melee fights. As they don't require PD, you get more PS, shield, IF, athl, or whatever, and you get enough wpf to wreck shit in melee.