Atum has Abi damage + Abi damage Cap up aswell on her Cooldown skill. Her heal gets an additional effect of dmg increase. Hercs Ability Master Stat also increasing Ability cap. And I did the same setup again on Dummy, which has 10 Def afaik and I reached 1.85m, so I assume that I'mm capped and it wont go higher then that unless I gain another source of cap increase.

It depends on your team if you can reach the same damage or not. Lets put my DMM team for example. You know who the star of that team is? Vishnu. The whole team basically is all about her and get her going. LAthena for protection is perfect for that, so she is still a choice over Atum in my regular line-up. And Vishnu is even more consistent when it comes to Bursting then Herc is. She is ready to Burst on Turn 3 (Granted +20BG from Michael to do so). And after that she basically bursts every 2 or 3 turns consistently.
Herc is only used to have 50% Def down in total. I would instantly switch her for Shingen or Sieg if I had another debuffer as Hime, sadly I dont have one, so I need her right now.

Lemme drop another example: My Thudner Team. Dear god, this is a dream for probably many Thunder mains. BaalU, Marduk AW, Nuwa, Dian, Tyr, Mammon. Full SSR top-notch team. Here is the kicker: The Grid is just way to weak do go berkserk-pew-pew-style.

Herc? Not even considering her, cause in this case Mordred has proven FAR more useful then Herc. This team is not fast but extremly sustainable, has Def down by Hime themself, so all I need is some stalling options and Mordred with her Orb Eater + BP makes it perfect. When I get Shingen, I may tinker around with her, to check out the limits of the sustain then, since I then have more speed for Dian, but unless that point, Mordred has proven superior over any other Soul.

So in conclusion: Not just content and Team is a factor, also your Grid/Eidos should be considered.

That is the very same for the MEX skills aswell, they are depending on content and team.