Lethality is not inherently op
Lethality just combos too hard with all the other goodies physical damage dealers get. Namely the most broken stat in the game since it became a legit, remotely balanced game.. attack speed. Attack speed is partly broken because it is usually combo'd with crit chance as well which is its own can of balance tipping worms.
Lethality has become an alternative to crits + attack speed and honestly that is a good thing. However that also means that anyone who is thriving from it probably isn't doing well because they are dealing sustained damage, its more likely bursty and ability based damage.
Lethality is game warping, meaning it drags the focus of strategies around too much whether its balanced or not, but it is the direction we want to go in.
for example:
You get a spellshield, a ton of AD, armor pen, magic resist and move speed all rolled into one item.