Why lethality is strong (with math)
I'm going to cut to the chase this is current lethality vs old armor pen that riot claimed was too good.
Old armor pen @ lvl 1 with precision, marks, and quints
22.5
Current Lethality @ lvl 1 with the same
32.5 x 60% base + 2.22% for lvl 1 target = 20.22
Hopefully you already see where this is going...
Old armor pen @ lvl 7 with a 
44.3
Current Lethality @ lvl 7 with a
52.5 x 75.56% for lvl 7 target = 39.67
Old armor pen @ lvl 11 with 
55.5
Current Lethality @ lvl 11 with
67.5 x 84.44% for lvl 11 target = 56.99
As you can see the new lethality out scales the old armor pen with 2 items, while losing next to none early on in the game. There really is no reason this should be the case as it makes the 2 item powerspike too strong. Not to mention you can ride this power into the late game for damage vs squishies thanks to the scaling nature of lethality.
