My impression was that Yell and Accumulate allow you to run circles around enemies and get tons of JP and EXP with no effort, superpowering you very early in the game relative to the story battle levels.
You can basically do this however you choose, e.g. basic attacking an ally + Chakra, and so forth. I would argue that because story battle foes do not level scale up when you exceed them, any kind of dedicated EXP grinding is indeed overpowered but the particular method you choose to do so is not necessarily.
Overpowered things IMO would be things the AI just never seems equipped to handle, like Blade Grasp, 03 Faith, or Summon Magic.
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"Getting lucky 100% of the time... it's called PRO SKILL."
-BlAcK TuRtLe