The Vierhänder is two Zweihänders crudely crafted together with a two-by-four and scrap metal to make a much heftier blade. ((And an awesome play on words))
The Blockbuster is an engine block turned flail thanks to an exceedingly sturdy length of chain.
As for being over-powered or useless I’d counter that point with these:
It’s my hope that eventually your enemies would have access to these as well. Like hostile mutants, Bionically Augmented Raiders, rogue militants or nether critters. So for every jackhammer tipped spear or belt-fed grenade launcher you find there’d be another one out there in the hands of somebody or something that wants you dead.
When NPCs and NPC settlements get ironed out you’d have hard targets worthy of a one man seige in the form of hostile factions. Answering manpower with firepower.
It would be a fun addition, one which I hope encourages people to think up challenges worthy of the sort of firearm it would take a hauling frame to tote. Becoming a muscle-bound man bear or cyber-commando seems like it would be better with a carrot dangling in front of you for the effort.