His main weaknesses have always just seemed to be lack of a dash/engage, and he fixes that up pretty nicely with all the speed he gets from himself, items, and allies(abilities and items, including Righteous Glory and Talisman for a major boost). Other than that I think the biggest problem is he doesn't deal much damage mid-late game without Black Cleaver/Youmuu's, and now against a Villain he does. He's allowed to be tanky enough to sit under tower and mock with emotes while being bombarded by the enemy before leaving and healing up in a minute or less. Even if he doesn't heal up fully it doesn't matter because he'll still be tanky as fuck.
He used to be pretty unviable because you could run away from him. Dash or whatever, and the biggest engage he had was from coming up behind you from the Fog of War, and wards helped that pretty nicely. But with his synergy with Dead Man's Plate and Youmuu's it gives him a pretty nice engage with his Q. If he saves his Q he can't really be stopped by slows unless they use 2 on him, so that makes the priority of the team to pretty much focus a TANK with CC over an average carry because this guy is in the middle of their team dishing out loads of damage on their carries and able to kill them himself.
So the opposing team has 2 options:
A. Let the Garen deal tons of damage to the team and move on to Garen's team's carries.
B. Focus Garen and allow the enemy carries to deal tons of free damage to your team, possibly and probably losing the fight.
I find it kind of bullshit that we have to focus down a tank. Even if we could peel Garen off of our carries that's still us, focusing the tank, even if not in pure damage but just CC and now we don't have CC for the enemy carries or to get through the rest of the front line.