Garen's kit actually does have complexities. Let's look at it one part at a time.
His passive only boosts his regen when it is active. Hitting him with one auto-attack or damaging ability places it on CD, reducing his regen. Granted at LVL 11 the cool-down to reactivate it is reduced from 9 seconds to 4, but the same rule of ONE Auto-attack or ANY damaging ability STILL places it on CD. He cannot activate it at will, he cannot reduce it's CD by buying CD reducing items, and, if you have something that applies Grievous Wounds, his Regen will FURTHER be reduced while it is on CD. Constant poke PREVENTS him from getting his Perseverance, and thus his Regen.
His Q, upon activation CLEANSES him of all slows currently affecting him. With proper timing, a Garen can remove a slow nearly instantly after it has been applied to him. But, it does NOT say he is IMMUNE to slows afterwards. Meaning, if you can get the Garen to use his Q PRIOR to you applying your slow, you can render the MS boost he gets to be ineffective. Consistent or repeated slowing effects, such as 
's Frost Shot or 
's Pillar of Ice can remove his speed boost almost as quickly as he can gain it.
Next, his W. For the first 0.75 seconds once activated, he takes 60% less damage and gains 60% Tenacity. For the remaining duration, he takes only 30% reduced damage. Through wise use, his W can act very similar to 
's Riposte. But remember, that initial Tenacity and DMG reduction only apply during the first 0.75 seconds. The remaining duration has no Tenacity and the DMG reduction is halved.
For his E, it applies 33% more damage to a SINGLE unit. Meaning, if he is hitting more than one unit, such as a group of minions and a champion, or even a single minion and a champion, those ticks of his E will NOT apply the additional damage. It means the Garen player must position himself wisely to get the very most out of his E.
Lastly, his R. It is an execute that deals bonus damage based on missing health, the amount dependent on the rank of the skill. (28.6% for 1, 33.3% for 2, and 40% for 3). The more health you have, the longer it takes for Garen to get you low enough to execute you, which if played around wisely, can let you occupy him so that other team-mates can come and aid you against him, or allow you to escape and prevent him from getting a kill, if your character has a dash or a blink or a knockback to boot him away.
Next, here's what the tool-tip itself says about the Villain passive: "The enemy with the most kills in the last 5 minutes is marked as the Villain. The mark cannot be assigned to another champion while Garen is in combat with the Villain. If more than one enemy has the same bounty, the enemy who received it first is marked the Villain."
The Villain passive ONLY comes into play at LVL 6 at the earliest. Meaning, if you can shut him down early and keep him from getting levels, you keep the Villain passive away for as long as possible. Also, it REQUIRES that a member of the opposing team has gotten some kills. If he is in a low-kill game, yes, someone will be marked as the Villain, but that also means that as long as that does not change, the bonuses granted do NOT apply to someone that it COULD be more effective on, reducing his ability to shut down that potentially more threatening opponent, until he finally kills his current Villain.
So there we are. Yes, his kit IS Simple. But, by looking at the tool-tips and what they entail it CAN reveal some complexities and mechanically intensive aspects. I NEVER said he had ALOT of complexities, only that he had SOME.
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