Lore-wise (and we can say curently ingame as well),  most current melee weapons are very ineffective in killing heavy targets/vechiles (have fun chop down a helbrute with a power sword, Emperor be with you).

 Two of the more common armor-effective melee weapons are the lightning claw and the power fist; as far as i am aware they aren't in the game at the moment. Are there plans to impolement them in the future? Power Fist is a long-time part of any Adeptus corps armament, and since a while, Lightning Claws are not Terminator-exclusive weapons either.