Lethality's Problem isn't Lethality itself, it's the Items
A lot of people have been complaining about Lethality, but I think it's more appropriate they complained about the items that give it themselves:

The actual mechanic of Lethality is an overall nerf from Armor Penetration. It's not the real problem. The problem is Riot nerfed Armor Penetration so they could give the items other tools. That way, assassins could use other tools than just brute damage to do their job.
Thing is these all provide a lot of what assassins need: High amounts of AD (each is equivalent to the top tier AD items), MS, even more MS (with yomuu's), a spell shield, and vision sweep. The added utility/stats on top of all the AD/lethality is what breaks this item line.