'Broken'? in a way, yes...though more a epitome of something that is broken in the game overall.
'OP'? Not at all from my PBE experiences, quite easy to kit.
Now the thing that is broken is the amount of tank hate he has in his kit, it's basically the peak of riots 'we don't do tank' design philosophie.
High %hp damage on his spamable Q, which on it's own is fine cause as a juggernaut with no hyper steroid and low target acces, he needs to be a threat to those he reaches somehow.
Even the truedamage nuke on the W is fine, it's avoidable and it requires him to take damage first in order to do any meaninfull damage. Though i'm certainly not a fan of %hp damage and truedamage in the same kit, leaves little option to built tankiness against him
However, what really is broken is his ult. 60% bonus HP of the target as aoe damage. This creates such a massive difference in damage depending on whom he faces.
His ult vs squishys has poor damage, but slam a regular frontliner like darius into them and it's 1500+ damage. Now add an hp stacking champ like cho or sion and his ult can just oneshot any backline. and i'm not even exxegerateing. I litterally killed an ashe 100-0 by slaming her cho into her, just that 1 R and nothing else.
This means that choosing a beefy champ if sett is open is basically helping the enemie team. If you built hp vs him, he benefits more from that than you and that's just a horrible design. He can't do shit vs the likes of fiora or riven, but if you play a tank, you might as well stay in fountain cause jpining a teamfight helps him more than your team.