Without commenting on his stats (which are always subject to change, like -5 movement speed on irelia) as we aren't familiar with Sett's power levels yet, it can seem as though his W is a great tool for trades. However, according to the wiki he doesn't become unstoppable and is subject to being crowd controlled. I assume this means that many champions can just stun Sett and stop him from dealing his W damage, which isn't small after long trades. His ult gives a 99% slow for 1.5 seconds (as well as tons of dmg) but it seems that his kit doesn't offer much combat potential other than max % hp, instead seeming like a more stat reliant champion such as mordekaiser. So does he just lose in lane to all the other juggernauts? Is his w interrupted by Garen Q? (I assume so)
Whilst I can see him doing really well against tanks such as ornn, as well as people that have no on demand stuns or knockups such as Tryndamere/Akali/Teemo/Vlad, I imagine that he would struggle against more prominent juggernauts and bruisers e.g Renekton Kled Mordekaiser Darius.
He's going to be really bad compared to other simple champions due to the fact that his W is so easy to play around: Go behind/Stun/Knockup can be accomplished fairly easily considering that Sion already exists (so people are used to playing against such abilities) along with the high mobility of many top laners. His sidelane is also quite meh as he is quite easy to kill in a 2v1 compared to darius who can turn on you with disgustingly high damage, or garen who can just run away easily and regen. And before you say that his teamfight is good - well duh, otherwise he'd be worse than pre rework urgot, remember that illaoi isn't a good champion, but has basically all the same strengths and weaknesses other than illaoi having higher 2v1 potential, being a larger damage threat, and sett having slightly more cc.
I may be completely wrong about this and find him outdueling mordekaiser and darius in sidelanes while also being a bigger teamfight threat than kennen a few days later when people start getting a hang of him, but until then, I probably won't change my opinion of his kit being bad - it's like he's designed for a fighting game rather than a juggernaunt for league.
Disclaimer: I do not own Sett, nor have I deeply investigated on PBE videos regarding details of his abilities