The real problem with the power weapons in this game is that they are very situational; you never really will need one until you're actually in a situation that calls for it. Meanwhile, even if you do wind up needing it the Mystical or Sidekick (depending on what you have equipped) generally does the job better without any effort needed on the player's part. It's just placement of the Mystical and/or relying on the AI.
So, you have these power weapons that are ultimately for really specific situations but cost just as much as a Sidekick or a Mystical; sometimes even more than either of those. This isn't even factoring in the amount of LP they require to be in your loadout in the first place (RPG being 5 LP and only getting one shot? Hello? Doesn't even lead to an instant KO. Meanwhile the China-Lake gets way more than one shell).
They would be way better as placements on the maps for sure. Players wouldn't have to spend LP or cash on them and there would be a reason to go somewhere and fight over a power weapon. UC3 at least had this much, it at least had a few power weapons on the maps. For all of UC3's faults in the beginning, it at least tried to maintain the "fight over territory where power weapons spawn" or "go to where the power weapon spawns, grab it ASAP, and make use of it". Reasons to go out of your way to go to a specific part of a given map.
If they aren't going to revamp the loadout system in any way (or rather, are unwilling to do so) then I think the least they could do is buff some of the heavy weapons. Smart buffs. Stuff like, say, getting two RPG shots with the first RPG you buy and then any subsequent time you buy the RPG you only get one shot. Or, just have the RPG KO automatically; no downs unless it's the splash damage. Otherwise, if you hit someone directly with it they should automatically die.