Killbow is too random in most cases, even against your ideal targets. If you don't roll that initial 6 to hit, you still have to roll a 3+ to wound, then they get a save against it (and most big hero monsters you're targeting are going to be save stacking to give them a not-so-negligible chance of blocking that one hit), THEN you have to roll 5+'s to add to the damage. At any point in this sequence you could just get unlucky on a single roll and lose out on your 130 point unit doing anything that turn. Also, it sucks against normal infantry. If it was like 15-20 points cheaper, it could be worth it as a scare tactic.
Otherwise, boltboyz are just more reliable against all types of targets. I like them in 6's because if they get hit with anything you don't risk 360 points evaporating instantly.