Want to know why Grasp and Conqueror are broken keystones?

Because they can be stacked up anywhere. On minions, on monsters, on champions and structures and then easily proc it on one champion. And they don't have a cooldown after they have been procced. You can proc it once then charge it up immediately then proc it again and so on.

What about other keystones? PTA only on champions. Lethal Tempo only on champions. Has cooldown. Fleet can be on minions but reduced effect. Needs to charge up. Electrocute on champions. Has cooldown. Hail on champions. Has cooldown. Comet, Aery, Phase Rush on champions. Has cooldown. Aftershock on champions. Has cooldown. Klepto, Glacial Augment only on champions. Has cooldown.