I've seen a lot of posts lately complaining about tanks and a lot of people complaining about penetration. normally i wouldnt care but i just came to a conclusion.
after building somewhere between 300 and 400 armor... i was still taking 3k damage from twitch.
This is a VERY damage heavy meta, and as it is there are few ways to counter it. Sure, tanks shouldn't be facerolling entire teams through sheer durability, but as it is I think it's a little unethical how quickly they melt. Especially when it comes to % health damage
Armor doesn't do nothing, but it really does stop being very effective once they've got any kind of armor pen
When you buy 
you're looking at about... 43 effective armor from it
yes, there is no point in building armor because instead of dying in 4 seconds you will die in 0.1 seconds
You now have have 300 bonus armor and crits only do 40% damage, which is all of Twitch's attacks.
If you really want to be tanky, you should focus on health, and not resists. This is because it shields you from both types of damage as well as true damage like red buff and elder drake buff.
