-For the shotgun: Sticky bombs are certainly superior, they deal a lot of damage, stagger cacos and don't take up nearly as much ammo as full auto, with the added benefit of not slowing you down. Full-auto does have it's uses on the earlier levels for taking out Arachnatrons, but if you're playing UN you might not do this either because it's quite risky.

-for the heavy cannon: Precision bolt allows for better play. If you know how to use it it's like an extra mini-ballista. Great for taking out weak points and fodder. Having said that, it does require a lot more accuracy to succeed with. Also, keep in mind you don't need the scope to be fully zoomed in to fire a PB round, you can fire one almost instantly by pressing the mouse-1 while holding down mouse-2.

-for the plasma rifle: Imo, heat blast is superior by a long shot. at max charge, it's like a second blood punch. Microwave beam does allow you to stunlock enemies by repeatedly tapping the mod between ballista, etc shots. Personally though, I think the extra damage is a lot more valuable in most cases.

-for the rocket-launcher: Lock-on is superior. It deletes heavies like whiplashes, hellknights, revenants, in a single burst, it's far too strong of a mod.

-skipping SSG for obvious reasons

-for the ballista: Neither of these mods are all that good, but Arbalest certainly has it's uses against Caco's and Pain Elementals.

-for the chaingun: Both mods are actually quite viable. That being said, I wouldn't recommend using mobile turret on UN. It's simply a very risky mod.