Greninja was very widely agreed to be broken in ORAS OU (82% majority). From what I can tell, the only notable checks that were introduced in SM OU are Tapu Koko and Toxapex. I'm doubtful that they're enough to change the outcome.
That's sort of my problem here. There's always going to be a couple of mons that are more powerful/centralising than others. We even divide tiers into viability rankings because of this. Are we really going to ban every single Pokemon that A+ or S-Rank? By the end of the year OU won't exist anymore and we'll jump straight from UU to Ubers.
^Hyperbole there but I think you get my point xD
While it's true that there will always be centralizing Pokemon, Greninja stands out as being fantastic at both breaking and sweeping. Usually if a Pokemon is good at breaking, it's relatively easy to revenge kill (see: Lele, Bulu, Hoopa-U, Manaphy, etc). Pokemon that can wreck both offense and balance/stall are hard to come by. M Meta and Mawile sort of follow under this category as well, but both use up a Mega slot, Meta is more predictable, and Mawile has to rely on Sucker Punch.
After all the bans, ORAS OU was pretty balanced near the end of its lifespan, where Clef, Lando, Latios, Keldeo, etc. were the metagame-defining Pokemon. Not many people thought they were broken.