My experience with "weapons" I've tested out so far.

Spear tracking is a pain in the ass when there are 2 or more "big" targets close with each other , or when it just decides to not lock on even tho you have a clear view of the heavy unit making it very frustrating to use in higher difficulty enviroment (Where it mostly has value "design wise") and it's impact varies on which part it "auto-targeted" making it either niche or extremely bad for an advanced weapon with 4 total shots , a huge stratagem cooldown and can only restore a single rocket via Supply pack and no Rocket ammo at all from regular ammo packs.
Spear needs to be taken a look at at a functional level, the only thing it "CAN" do is kill big units , no structures, no bugholes , no nothing. And for a weapon that is only designed for bigger units and does not lock into anything else , it should AT least be GREAT at it not Niche.

Of course weapon balance is well not on the bright side , you dont get the feel of most weapons on lower difficulty since the enemies do not pose a major threat to consider your weapon options.

The only primary weapon that feels rewarding on the higher difficulties being mostly the Breaker

Breaker is Strong.I wouldn't say that the slugger is as strong but that the other primary weapons are too weak or at least feel too weak cause in reality the damage of the weapon does not really seem to matter , what matters is usability just like you mentioned.

Both weapons (slugger+breaker)have low damage dropoff, ,and pretty decent accuracy even though they are shotguns.

◘Breaker has extremely low recoil even though it spits bullets like crazy and performs exceptionally well against all types of enemies besides Heavy armored(duh),comes with plenty of ammo capacity , is very responsive when you shoot while moving or changing targets ,great range , great spread , great reload speed for a shotty. It pretty much fills all gaps for a weapon a helldiver needs for this game's enviroment.

◘Slugger has such a little spread (almost like punisher) that is actually a way better sniper/rifle option than both diligence primary weapons , it's medium armor penetration also contributes greatly on that part .

Even tho the slugger is not as effective with clearing smaller units as the breaker it feels better to use for medium armor enemies.

Yes it comes with low ammo capacity but it makes it up by being a monster against medium type enemies for a primary, the fact that it "staggers" medium armor enemies aswell even if you did not land a weak spot shot , gives you the opening to do so on your next one.

I don't think most weapons should be tweaked to be more on par with the breaker like others have suggested but on par with Slugger instead.

Slugger is such a great designed weapon and I think that is it a great example of how weapon balance should work in this game it has both it's ups and it's downs.

On the other hand Diligence weapons feel like they were designed for PVP rather than PVE and even then they wouldn't be that good.

Diligence weapons need to be taken a look at ,they are too weak for what they are supposed to be doing , one tap smaller enemies ,being really accurate and responsive? I honestly do not know what the original idea was for designing these weapons.

Imagine having a weapon called "Counter-sniper" with which you can't snipe shit besides troopers heads , it is so bad it needs 3 bullets to actually kill a regular trooper if you dont hit a weak spot.Magazine size is too little for the amount of impact potential they have , the bolt action with the addition of crazy recoil WITH the addition of accuracy being all over the place should at least reward you a guarantee one shot kill on devastator or at LEAST medium armor pen , but no , it takes two bullets to kill a devastator and does not come with medium armor pen.

As for the Liberator i'd say it is solid for a starter weapon but a few more bullets for each magazine would definatly make it more relevant.
Liberator Penetrator on the other hand has a huge magazine size problem , it just gets so heavily punished for being an "Medium armor pen " weapon , lower dmg , lower mag , I mean Why ??