Of course this is opinionated but it seems that the attacker hits too much, since I performed the scenario of a rune-zerker against a 1-defence pure (with defence bonuses with black d'hide boots, etc) and it hit 10/10 times repeatedly.
On RS (obviously, it's chance - but for the norm) you'd expect to see a rune-zerker hit around 7-8/10 times typically