We may or may not had this discussion before, but in the case you did not know. Zed's kit now currently is in now 2 spots.
1. You thrive because you bought cost efficient lethality items against a person who bought lost chapter and you kick them in the teeth.
2. You cannot since the enemy has a anti-zed rune page with 2 armor shards and bought seekers AND tabis.
(And since you are Zed mid, you are like 3 AD team comp. This is a totally valid buildpath.)
So, lethality. It gains more armorpen if the target has lower levels than you. And is balanced solely through that math. With easy thinking, you will see that this stat heavily focuses on ganking that low level ADC. But if you are forced to fight even a moderately tanky enemy 1v1. Things starts to get out of hand because the phenomenon below comes into effect ALL AT ONCE.
1. Your lethality stats are negated by armor.
2. Lethality items for the sake of balance has only 55 AD and you are not a DPS.
3. Your cannot snowball off them since you are even on levels.
4. Your enemy bought 2 amp tomes so you are in the risk of being trolled because you cannot find out if they are going armguard or lost chapter.
So to avoid this situation.
1. You buy hexdrinkers and get nullifying orb.
2. You get a even standing on mages because you cannot kill each other.
3. It becomes a strategic push/roam match. But you on the disadvantage since you are melee.
I strongly believe lethality is only good for "cheesing" and I think it is bad for the game if cheesing is reliable, or your pick is ditched by the math of the game because you can't pull off the cheese. It creates a situation not fun for both sides.
Even in the case for other lanes, such as lethality MF. You guys have probabaly seen duskblade tipped MF bounces that rips off like 70% hp of a carry. How is this fun?