I don't think it's overpowered, unless people actually take advantage of the chain effect, which puts them more on par with the most game impacting LBs if they're pulled right (Phalanx, a multi hit zantetsuken, Skyshatter that lands on unguarded stacks, Bahamut, BLM, etc), which is considered over the global results of a game yes. I just think however, that on its own it's not exactly hard to drop somebody to half HP and kill them. Heals can be a variable, but that's always hiding being perfect counterplays that ask a lot more effort than just pulling a Seiton.