I have no clue about the bullet sponges part. The monsters weren’t changed in Atlantis that I could tell.
The areas themselves are all completely side quest material. The quests only provide items or XP, no state points.
There are new monsters and new items, but nothing like the new throwing weapons from Ragnarok.
The biggest changes are that you access to the gambler that provides you with random loot for a gold.
The ability to go to put 40 points into a mastery and unlocking 2 Level 40 skills for each master (Nature has a cool one).
And the ability to fight endless monsters in a cave (each stage, the game generates a random set of rules, like no Potions or no active skills or no Regen).
I think, for a couple of dollars, having access to the level 40 skills is worth it.
The new areas are skippable, but I like the extra lore. It’s not as good as the Ragnarok lore though.
Day one purchase/full price… No. Under 5 dollars? Yes.
https://www.titanquestfans.net/index.php?topic=952.0
This random person reviews the new skills.
As for this DLC, I think I will wait for the next sale.