"Your http://fallout.wikia.com/wiki/Melee and http://fallout.wikia.com/wiki/Unarmed attacks do more damage, but less critical hit damage.
More specifically, your Melee and Unarmed damage is increased by 20%, but your Critical Hit damage is reduced by 60%, but only for those two weapon types. Since you inflict more damage consistently with Heavy Handed though, Critical Hits are less of an issue, unless the player relies on http://fallout.wikia.com/wiki/Sneak_attack_critical as a major source of damage or if the character criticals often due to perks, Luck, and high weapon modifier (certain perk/weapon combinations give you a guaranteed chance to critically hit)."
If I am reading this right, if you attack while sneaking you will be ok.