I was using it today because I didn't feel like moving stuff around on my characters.

It certainly looks like it has the highest DPS of any primary weapon. Captains and Servitors go down very fast. The key is to use CQB and not aggressive ballistics. For a long time I thought that CQB wasn't a good choice, since it kills impact. However, the stability increase is huge, and you can get critical hits much more consistently. And when you are getting critical hits, you are doing 2.5x more damage to non majors.

So why does it suck?

PvE - It's a combination of the low magazine size and just the general role of the auto rifle. The low magazine size means you are always reloading, which is a drag. But the biggest problem is that the archetype doesn't offer any benefits as a primary. Hand Cannons might not have the best DPS, but they kill NOW. Scout Rifles kill from a safe distance. So Necrochasm has no benefit over a hand cannon for mobs, and while it shines on tougher enemies, you would pretty much switch to something else like a special for those. Why waste my exotic slot to kill captains, when I can use found verdict and do it better?

PvP - Yes it does a lot of damage, but you can't kill with it. There's a reason that higher impact slower firing weapons are popular. While missing a shot on those types of weapons is more costly, it's also much easier to not miss a shot. You have more time to adjust between bullets. So while Necrochasm has a theoretical lower time to kill, you just can't land all the bullets to do so as easily as something slower firing.

Solutions:

It's going to be gimped no matter what you do because it's an autorifle. Write it off in PvP. In PvE, a stability increase across the board, plus either reactive reload or a bigger magazine would do wonders to make this an option. The cursed thrall explosion should also trigger 100% of the time, and immediately like firefly. Like I said, in a straight up contest between primaries Necrochasm actually does really well with CQB. But when you take the whole picture in to account, and realize you have special and heavy weapons too, it's not so attractive.

No perks will fix it. Max the stability and raise the mag to 70. Fixed gun.

It's what you said but shorter.