So, I Just Played Versus Mundo.
I completely understand why he has a 57% win rate. The Grasp of the Undying synergies so well with his new E. Legit, he is dishing out crap loads of damage while being super tanky.
It's not because lelelelel all AD teams, or the Grevious Wounds LW isn't good. No, it's because he is doing carry like damage while being a super tank.
I died at level 2 vs Mundo as Sion. I understand, that I don't do a lot of damage as Sion early but why should he?
tl;dr masteries synergize too well with his kit making him do tons of damage while being tanky.