He gets the ability to secure kills on the Villain, someone he can't control who gets that. So it's really good on whoever is fed, allowing a team to make some comebacks without being stomped out and put down. Sure Garen does well himself, he now has damage like he should. Garen has no hard CC, his Silence got nerfed, his movespeed ability nerfed, his ultimate is only usable on one person in a fight, and his judgement is his only major form of damage, and it doesn't do a whole lot against someone who isn't the villain.
He is easily kitable, and doesn't off much to secure a win on his own. He can be crowd controlled and bursted, or kited and avoided until his team cleans up everyone else. Sure he barrels into your team to get the Villain, and you let them lead him out of the fight and get an easy 4v4. Not to mention he has several lane counters too. Sure he got a huge rise in win rate, but did you think maybe that has to do with him eating current meta picks, or being picked a large amount?
I played Garen before the rework, and I won plenty of times with him. I've also played against him post rework and watched him crumble.