Lethality, if full stacked, is only capable of negating base armor values.
Just like magic pen, if full stacked, is only capable of negating base magic resist values.
That's not broken, they are both functioning as intended as stats that allow you to do max damage to targets that have no defensive items or abilities.
What's broken is some people's ability to adapt.
You can completely shut down someone using a full lethality build from being able to burst you down by simply incorporating a bit of armor and health into your build.
But no, they want to keep stacking max AD/AP and think they should be entitled to have no downsides against someone who has specialized their entire build around blowing up squishies at the cost of being unable to deal with moderately armored targets.
I've posted math here recently that backs up all that.
But I also know it's true from experience because as a Talon player I am acutely aware of how various levels of armor interact with lethality - even moreso than many other fighters/assassins because Talon is one of the most pure assassins in the sense that he is 100% all about his burst and has little to fall back on if that can't kill the target. He's very all or nothing in that way. As a consequence, it's extremely noticable, moreso than with other fighters, when his lethality build is being frustrated by even small amounts of armor. It's also abundantly apparent when moderate amounts of armor and health completely shut him down. And trying to take on a full tank by yourself is almost always impossible even with dominiks.