Part of the problem here is that these weapons, the Plasma Cannon and the starswarm of the Reaper Launcher specifically, are meant as anti-infantry weapons. In TT and in lore the plasma cannon is used to wreck anything in cover, wearing power armor, or both. I'm not sure about the Reaper Launcher exactly, but I assume it is similar. Besides, the devastators all ready have AV in the form of multimeltas and lascannons, while the Reaper Launcher functions well in both roles depending on the fire mode. I do believe both of these weapons should be adjusted slightly, but should retain their role as infantry portable artillery rather than turned into just another way to kill a tank.

We don't have enough well done cover for them to be necessary...seriously when have you ever thought "Oh man, how the frack are we gonna get those dudes out of their camping entrenched spot form where they fire?!?!", id say never? Its because there are usually more then one approach to any spot, even if its small, you have assaults for getting people to run out of cover. We have grenades, etc. etc.
I firmly believe these weapons need to be completely reworked or just removed. We have enough stuff that totally kills tanks now (meltas, fists, grenades, las cannons) and they arent exactly super durable as it is, we don't need another anti tank weapon. I have no real issue playing the game as is if they just get gone.
They could add a little better sniper rifle then for instance Stalker Bolter to each class, to hit those dudes high up on the wall on that one map.

I've had that happen quite often when the defense is not just ditching points they've captured.

You do know the stalker bolter is LSM only right?

Nope, that was nerfed. 80 dmg chest shots 160 for head and you get 8 shots, damage falloff is included as well.

I have felt the need to lob ordinance at entrenched positions multiple times since it takes less time then preparing a flanking assault. One of the best ways to deal with an entrenched gun line such as what you would find on Maggon is to lob plasma or shells at it rather than throwing enough bodies at the position that they run out of ammo or hoping the defenders forget to cover a door(protip: they usually don't). Grenades have a tiny kill radius for the most part and are limited by their ability to be thrown by hand and are kinda wonky at the moment, while the PC and Reaper Launcher give the user range, power, and more then one use . Ultimately these weapons should just be re-balanced(longer charge time or smaller blast radius) in their current niche of being portable artillery rather than discarded entirely rather then being relegated to a non-lore friendly role of AV(or outright removed).

don't drag the poor ork rokkit in here, when was the last time you saw an ork army spamming tons of rokkits? Never, because it's bad (granted most ork ranged weapons are bad so you may see a few) it's single shot, has to be reloaded AND charged with every shot, and doesn't have any partial charge attack like the plasma cannon or such. Yes it can one shot players, if it doesn't go flying off in some random direction if you're standing more than 10 feet away from them.

Actually, no, I have seen them deployed frequently and in quite effective manners by multiple people in a team instagibbing everything that went past them.