Maybe I’m just bad, but it feels like they are very inaccurate and/or do very little damage.
If you mean the shotgun ones than I find it’s more on how you use them. I find slugs are better at taking out exposed components aposed to buck shot for getting rid of the armor on it. If your aim is good they are great at removing weapons off hunters.
Shotty slugs are quite situational and rare to find as well. I’ve tried them but i prefer buckshot more for the spread-out damage. Birdshot gives even more spread but single pellet damage is too low for my taste.
Having choke on shotty, especially 5* one, makes slugs very accurate.
In my opinion, slugs are the worst ammo for the shottys. They are useful if you don’t have anything else but buck and bird are better. Same is with e.g handguns. Starting Möller is good until you find Klaucke.
Are there any other slugs in the game besides shotty ones? 
Slugs requires you to get extremely close to be effective against machines. It is better to use a melee than the slug shells.
No there’s not but it was more a point of clarity as new players might not have seen them yet. I first saw them at the airfield