Others have already said plenty. You can check my posts for big huge detailed comments about shotguns from before if you like.
Shotguns aren’t as simple as just headshots, and as for slow reload and not enough dmg that’s just not even remotely true. There are other ways to build and especially for psykers after the itemization update is out, but here I’ll assume the shotties have Man-Stopper and Scattershot.
Here’s a picture from my notes to help:
Just to make sure you know: any finesse dmg buffs (finesse = weakspot, crit, both) apply to the portion of finesse dmg only. So if a weapon does 100 base and 150 weakspot, +10% weakspot from talents etc. is only 10% of that 50. In the case of shotguns ofc. that means something like Agri with low base dmg gets almost exponentially more powerful than say Kantrael, the more of those finesse buffs you stack on top.
So ofc. it may seem like all of them but especially Agri & Lawbr are ADS & headshot guns only, but a difference as large as Kantrael’s base dmg is not so easily overtaken. More importantly there’s the issue of spread, breakpoints, and Scattershot. Hitting one mob in the head killing it & losing your Scatter stacks is a heavy price to pay, when 1-3 hip shots could’ve killed it and 10-20+ whatever elses were around too instead, each capping Scatter again and again.
This is why all 3 shotguns are much more complex than just about any other gun. You really need to use hip, ADS & specials to reach their real potential. Every shot needs to consider what comes next, whether your Scatter is stacked or not. The moment you lose Scatter, you lose crits, and when you lose crits you lose both a huge portion of their damage and that infinite cleave from Man-Stopper. And therein lies their balance.
Agri as fantastic as it is for weakspots struggles sorely with stacking Scatter bc you need those mobs in a line to hit them. Losing scatter in the middle of a fight means dealing a fraction of its potential dmg with several shots wasted until you get the cleaving crits going again. So often enough it’s better to fire through the chests of at least the front row mobs, sometimes even with hip fires despite losing a ton of dmg when it does far more overall dmg and keeps the Scatter going.
Agri’s special for example is often best used to cleave through whatever line lets you hit as many things as possible. Bc despite its fantastic CWS dmg, that pinpoint accuracy easily loses the Scatter. Yet its raw dmg is so insane it can easily kill a line of just about everything with just the crits alone. Now ofc. I’m not saying Agri isn’t a weakspot weapon, it absolutely is. But it’s not all it is.
Lawbringer specials with its perfect horizontal cleave is ideal for spread headshots. But it’s slow, does low dmg (this gets addressed in the next patch btw) and has poor range. So you’re always forced to balance between it and regular shots on a case-by-case basis. But as the hybrid between the two its hip spread is already pretty nice. So switching between spread & Scatter > ADS headshots > repeat plays a much larger role with it than Agri. Ofc the spread shots soften things up so nicely those headshots are even less important unless you’re shooting for long range.
Compared to those two Kantrael is ridiculously easy to use. A single special from about any distance always stacks Scatter on account of its spread & cleave, and its base spread is so wide it’s harder to NOT stack Scatter no matter your distance anyway. Kantrael shines specifically because it doesn’t need to care about headshots - often not even ADS -, it will still kill about everything in the general vicinity of whatever you’re pointing at. In fact, its dmg and spread is so high, and its finesse bonus dmg so low, that most times you’ll do more harm than good if you even try to go for heads. It’s part of why I like it, an easy chill choice when you just want to ram right through in a chill game. 
Anyway tldr; Yes, shotty headshots do more damage just like they do on every single other weapon. And yes, especially Agri is highly reliant on them. But shotgun’s real power isn’t whatever they do to one head. It’s about crits & cleave and how losing one means losing both. Their real priority is practically always cleave & Scatter first, heads far second. Sure, sometimes you can do both and when you can that’s your obvious best move. But almost more often than not, that’s not going to be the case.
Edit: I realized I again just yammered too much and made it too confusing. So I just want to get back to this point. →
There’s 10 specials & elite in a ball. What’s the better move:
- Aim for the heads, each shot killing what it hits but they only hit a few each. Meanwhile you lose Scatter, lose crits & cleave, while the mobs move to spread around or shoot you back?
- Just blast the middle of the ball 3x and done? Including whatever was in front or behind them too.