My discoveries thus far
Garen has rather weak armor and normal MR base stats
But once you add his passive he has meh armor and pretty good MR base stats.
But then you add additional resistances and his passive 20% armor and MR seems to do much more work.
Ive also noticed that his HP regen passive scales with HP but isnt too useful in battle and his ult is strong but useless outside of battle.
Basically theres two contrasting sets of abilities.
Obviously there's passive vs Ultimate, Ill get back to this.
The other is armor vs MR itemization.
Garen has a speed buff and speed debuff remover, AoE, a silence and an execution Ult. Assuming he's a strong persuit champ I wondered, what would he build?
Lets look at some nice antiphysical itemization for him assuming he needs some good sticking power and all that.
frozen heart (gives mana he can't use)
iceborne guantlet (provides mana garen cant use thus wastes gold)
Now lets look at his antimagic itemization under the same role assumptions.
banshees viel (prevents CC doesnt actually help garen get to the target faster and provides mana garen cant use)
Maw of malmortius (helps him live a bit longer but doesnt actually give him more mobility advantage)
Spirit VisageSpirit Visage(helps him heal and reduces the cooldown on his speedboost by 0.8 seconds)
as you can see, some of his strongest options against physical damage (towers, most carries, and what you're gonna eat alot of in most teamfights) are things like
maybe sunfire if he feels like some extra AoE damage and needs to damage as he chases to help with his ult.
His better options against magic that help with his task are basically
This is just my opinion but the anti physical seems alot more versitile than his antimagic itemization.
Now lets look at the neutral itemization.
Locket of the Iron Solari
The Black Cleaver
of these maybe frozen mallet and reverie (which gives mana regen) and warmogs would matter much against magic users.
My own conclusion here is that garen has poor itemization against magic users but can in fact build against them but he has no fullbuild option that can deal with both magic and physical at the same time AND give him mobility advantage AND significant damage. This in combination with the fact that he must build armor before level 18 if he doesnt want to have some of the worse armor levels in the game and the fact that resistance reduction probably has increased effect against him would mean that garen's kit probably has a counterstrategy perhaps any champ can exploit. He scales with many different things but all his scalings are essentially connected. HP and MR/Armor are very tightly related valuewise.
Now about that ult vs that passive. He cannot use his ult outside of battle, he cannot very easily use his passive in battle unless he builds HP and thus can heal a bit more with his passive which would effectively weaken his second passive which gives him 20% armor/MR. On the otherhand if he builds armor his passive is less useful everywhere in general. With armor he can more easily deliver his ult ftw, but he cannot do much about magic damage. With MR he can survive magical assaults but he will likely have less power and mobility for the most part unless he builds something with armor which doesnt help with magic and even then, his armor wont be at especially high levels if he doesnt build plenty of it and his DPS is already raily low without items to back it up.
My overall conclusion is that garens abilities are each dependant on itemization and may be counterintuitive to one another thus garen is at the mercy of his itemization and the enemy team comp. His weakness is team variety and build based hardcounters.
Garen has an affinity for defensive itemization and must give up slots to keep a reasonable mobility and power. He is dependant upon his W for sustain in battle yet is also more vunerable to any form of sustain reduction such as grievous wounds, anti AS, antilifesteal in general etcetc. Not only is his damage as a melee fighter average at best, his natural AS, which would require even more of what few of his 6 slots he has left to improve, is not anything impressive and he has no built-in offensive steroids to speak of.
In short, counterbuilding garens offensive capabilities is not expensive at all unless he's fed beyond reason but his defensive capabilities are another story. He does have his spintowin (range 330, just short of twice his aa range) and ult (range 400) but both are low range and have alot of down sides damagewise.
I haven't done the math but I suspect % hp damage of any kind, resistance shred, persistant mobility debuffs, and antisustain (armor > hp against lifesteal) are the easy answers to garen
Garen can be countered by understanding itemization. What you think you know about itemization is pure assumption which is how you get left in the dust when itemization goes through changes of any kind.