I have tested a lot of slugs, mostly for 3Gun shooting, but some for hunting and some for LEAs as a consultant. In my notebook, I have over 50 kinds of 12g slugs fired from 20 some different shotguns. Add in the choke factor and that is a lot of data. I can tell you that data won't give anyone a warm feeling to help them pick the right slug for their shotgun. I have had sister guns not shoot the same slug to the same POA/POI and vary group size by as much as 100%. Usually, with fiddling with the chokes, you can find one that will make a combination passable. A shooting buddy picked up a case of a certain slug from me a few months ago, one that runs really nice with small groups in my M2. In his M2, they were awful. So, he gave me back what he had left and tried something else and viola, good to go. What his M2 liked, mine does not like.
Slugs spend enough time in a, relatively, flimsy long barrel that a lot of things can affect the accuracy, POA/POI. In most cases, "benched", "supported" and "offhand" will change the POA/POI of slugs. Due to the shooters hold and recoil, what groups and hits POA well for one shooter might not be the same for another shooter of a different stature or with a different grip on the gun. 35 yards, not that big of a deal, 100 yards, we can be talking a foot of change and even more. Another factor is vent ribs. Some have taken to slicing sections out the vent rib to get a significant increase in accuracy.
BTW, 20g is easier than 12g and lightweight slugs are affected less by the issues that affect shotgun slug accuracy. I was pretty sad when Remington discontinued the frangibles that were 5/8 ounce at 1650 fps. Those were lasers in almost every shotgun I tested them in.
Shorter barrels, no vent ribs, IC chokes, lighter slugs are all attributes that tend to produce smaller groups and have better POA/POI correlation. Measurable by several inches at 100 yards.
So yes, Prof Young, they can make a big difference. But, I'd not say it is a brand/load issue as it is an issue with smoothbore shotguns shooting slugs in general.