I find some things about Sett a little ridiculous that could probably use some tweaks.
One. I believe the shield he gets from his grit is a bit extreme and not the most balanced thing that could have been done, the shield ontop of the slows and damage he deals makes him very difficult to deal with early and late game even when you are really ahead or he is really behind.
I think the way Sett is set up he leans toward being a massive damage dealer while also being a huge tank and I think that he should lean more towards just one instead of both. I don't find him to be all that balanced at the moment and generally the only time I see a Sett fail at dominating is when the person playing Sett just makes a bad play or a lot of bad plays, so I feel someone who has practiced him a lot or someone who is generally really good at the game (like a pro) could be near unbeatable if they just play him.
I also feel that his passive damage could use a little more power late game as it seems far less useful late game, and if he wee to gain more damage on his passive late game and lose some power in his shield and ability damage he would be overall a better champ.
I also think there are some bugs with his ult where it doesn't quite activate properly when you use it and I have noticed some players getting upset over it not activating when they attempt to use it.