Except that you're leaving a lot of details out about how exactly Sett has to work with those sort of 'perks'. His stun has to be achieved by getting an enemy on both sides of the AOE which, outside of laning phase, is not as easy as it looks. I can attest, as I have already played quite a bit of him on the PBE.
9/10 times when you use your Ult, you're going into the enemy team and will be focused, making it a huge risk factor. Hell, it's not even a great engage due to the incredibly short range. It makes him incredibly easy to kite, he's terrible at engaging in teamfights because of this.
Yes, you have a huge shield and true damage from your W, but you have to take damage to get both of those to reasonable amounts and the shield depletes incredibly fast, making extended trades extremely disadvantageous.
Again, for his HP regen to even be noticeable you have to be low on HP, making him especially susceptible to executes which happen to be pretty common these days.
Sett is only really good at fighting other melee champions, primarily tanks. He's extremely easy to kite out in many scenarios, even by bruisers like Irelia or Kled. The only thing he excels at right now that we can say for certain is that he's great against (most) juggernauts and tanks.