Are there different use cases here? Seems to me shred is better choice all the time?
IMO, it depends, while shred with Explosives, the Deceptor or Harrower is very useful, I personally find the 15 piercing of the Piranha more useful then the 2 shred of the Bulldog (the rest is the same). Also any single shot weapon would maybe not very effective with shred, but piercing is pretty useful especially in the later game.
Acid is very special and I have not much usage for it, because the effect is delayed. But if it is really much on big targets or also against the Pure (bionic augmentation receive double acid damage) then it is not really bad.
I realised I never said thanks for this too! Jormugandr cannon ended being my favorite weapon against Scyallas with rage burst once I understood how acid really works.
Agree on the nuance of piercing being better later as shred takes a while to shred armor anyway.