Well, let me preface this by saying anyone offering an opinion on how strong/weak a champ will be on release is a complete and utter moron.
Now, I will follow up by saying that Sett might be bonkers out the ass busted in all levels of play right now BUT he CAN be balanced without kit changes.
In comparison, irelia akali aatrox yuumi sylas, have all had tools taken away from their kit.
So, in the long run, I do believe sett will not be as much a balance nightmare as those.
In my humble opinion, it is understandable to release champions with numbers that will need to be nerfed, than champions that will need to have their Kit/Combos changed, because a champ with nerfed numbers is played the same way, meanwhile a champ with changed kit needs to be PLAYED differently, which will be way worse for people that invested time to learn mechanics.
Again, Sett is the no1 target for Bans atm imo. 56% winrate in masters+ don't lie