When it comes to the GS it’s shield is better than the tower shield both by pressing block and using charged heavys because of “guard point” it blocks 50% additional stamina damage just after pressing block or starting a heavy attack with the shield and also applies rend. The fact that heavy attacks count as guard point is baffling. My tower shield has 50% block stability while gs has 34% with the 50% added thats 52% or so but oddly enough the heavy attack shield blocks more than this but I don’t see this in any of the descriptions? I believe that it adds an additional 50% block stability during the shield phase cause during a normal gs block a heavy hammer hit does what it does to a tower shield but during heavy shield mode it’s half. Even a full heavy hammer doesn’t do 40 stamina like it should on that heavy shield and the window for guard point is very forgiving. This “guard point” on heavys also procs the 15% heal after blocking perk. Lastly, the window to stagger/stun between heavys is inconsistent cause of de-sync and forces you to wait till the 3 hit heavys are over before you try to sneak in a quick stun.
As for onslaught the damage is overtuned only for auto attacks which is odd, your auto attacks do double the amount of damage than your skills actually do 
honestly makes no sense. As a full heavy with physical favored gear at 2553 physical and 1847 elemental and 45% fortify with my slash resist I still take 2300 damage from the quick heavys, in 3.5 seconds I take over 7k damage from 3 heavys plus bleeds yet skills only do 600-1100. I think it mostly upsets me that the gs is out tanking me shield wise by destroying my shield stamina in 3.5 seconds with 3 quick heavys faster than a war hammer does when I’m heavy with 10% sturdy and 15% block stability from heavy but also being the highest dps.
Quick video of me being melted with 45% fortify big fat physical hippo boi in 13 seconds. S&S/WH, cant beat the GS BIS - YouTube
Is S&S in just a bad place shield wise compared to gs? Other than connecting with bash I feel like I have to run away from gs as if I’m some light armor user