Why is Armor Penetration even a primary stat?
In most games, getting penetration, penetrating bullets, taking the penetration talent or whatever. Is usually a tradeoff, a choice. It is something you do so your damage is not mitigated as much by heavy targets who have defenses, but in exchange you usually lose damage, rate of fire or some other stat for it.
Yet oddly, here in LoL, Riot has decided Armor Penetration is a primary stat like AD, AP, HP are. This doesn't make sense to me, I am not sure why any class, including assassins, should be designed primarily around building armor penetration as their primary stat. This entire concept is just begging for disaster in balancing the entire armor and tank system. Armor penetration should be a choice buy people pick up when enemies have too much armor and it is mitigating their damage too much.
If any champion/class in particular requires armor pen to function in all scenarios, maybe you should consider you know... adding it to their kit. Or perhaps lethality/armor pen should be added to their base stats.
In my eyes, purchasing Armor Penetration should be like purchasing Magic Penetration.
There should be some options for shred
An option for % reduction so tanks aren't OP
And an option or two for mild amounts of flat penetration

Leave these as optional buys for when the enemy stacks resistances or squishies actually buy a defensive item. And that is it. Rebalance these items with the lack of flat armor pen in mind. Rework/Remove EoN into a less toxic item and remove/repurpose Duskblade. Rework Wit's End because it is mostly crap, make it the AP version of BC, keep it useful for AA champions but maybe make it useful on casters as well. Add flat pen/lethality to assassins who really need it for their kit, so they can be individually tuned.
I don't know if everyone necessarily agrees with me, but I do feel armor penetration and magic penetration/shred both should be limited to "a choice" when the enemies stack resistances and a utility that champions who regularly buy
/
bring to their team, and should not be a primary stat that any class buys and relies on. If assassins need some incentive to buy non-crit items, how about looking into why crit items are a problem and why there are less pure AD options that outclass them(other than lethality) these days.
Personally, I miss building AD for the sake of AD.