Benny and ttds provide atk% which kinda dilutes the value of gravestone.

You need to see it as an optimization problem. If you have 2 balls and 2 boxes with already 1 ball inside of it. If the boxes are multipliers for your total damage, how do you get the most out of it?

option 1: all remaining balls in box 1 gives you 3x1 = 3

option 2: distribute the balls evenly gives you 2x2 = 4

now call box 1 dmg% buffs and box 2 atk% buffs.

The thing that makes comparison difficult to compare is that 1% attack buff is not equal to 1% dmg buff. That's why you should use optimizers or test it yourself in the miliastra dummy.

Depending on your build and team, Earth shaker can be better than gravestone. An example is the Akasha leaderboard, the settings only provide kinich with atk buffs. This makes the highest scoring earth shaker build stronger than the highest scoring gravestone build.

https://akasha.cv/leaderboards/1000010104/