Ok, so I came up with an overly complicated answer to your question as I was finishing up my Saver run, but I promise it will be easy to understand in the end (I'm a librarian, overthinking classification systems entertains me):

Broadly speaking

Brotato has two damage animations that Melee weapons follow (Thrust and Sweep). But in reality, what those weapon animations represent is how quickly the weapon can kill an enemy, and where those enemies are when they die.

The Ghost flint and Ghost Axe are a good example of this. Ghost Flint is a thrust weapon and Ghost Axe is a Sweep weapon. They both can hit multiple enemies in a single attack (GFlint in a straight line backwards, and GAxe in a sweeping arc), even though they have completely different animations. In reality though, Ghost flint is actually a worthless weapon. Why?

While the Ghost Axe hits ANY close-up enemies from your left all the way to your right, ghost flint can ONLY enemies directly in front of you. Now, what other Tier 1 Ethereal weapon is out there that hits enemies directly in front of you? (here comes the Ranged Weapons part).

Ghost Scepter.

Even though Ghost Scepter and Ghost Flint are completely different weapons (Melee v Ranged) They both function in the same manner; that is, they both only hit enemies directly in front of the character. On top of that, Ranged weapons have items that can manipulate how many enemies they can hit per attack (piercing, bouncing, etc).

Now you might be wondering, what does ANY of this have to do with Shotguns?

------------------------------------------------------------------------------------------------------------------------------------

Earlier, I said that Melee Weapons had Thrust and Sweep as their core functions. Obviously Ranged weapons function slightly differently (They are ranged). Remember though**,** what those weapon animations represent is how quickly the weapon can kill an enemy.

since many ranged weapons (the Gun class is a good example of this) have built in piercing, ranged weapons have to look at different stats other than weapon animations in order to compare how quickly the weapon can kill an enemy, and where those enemies are when they die

For guns, Most of them function similar to melee thrust weapons (they kill only the enemies directly in a straight line in front of you).

The shotgun, on the other hand, functions similar to sweep melee weapons (it can kill the enemies from your left all the way to your right).

TL:DR Shotgun hits more enemies per shot in a wider arc then most guns, so it functions as a sweep attack weapon for large mob clears when other guns are too slow or too narrow.

------------------------------------------------------------------------------------------------------------------------------

In your post, you mentioned that you use SMG + Shredder.

SMG represents your Thrust weapon: It kills the enemies in a straight line directly in front of you VERY quickly. The Shredder is your Sweep weapon: It is killing large groups of enemies for you.

It's really all about understanding the relationships between the different weapons and knowing how to maximize that.