Last Whisper Items are Essentially better than Lethality for Assassins(in terms of dmg) check it out
50 AD 35% Bonus armor pen and Grievous Wounds for 5s
60 AD (soon 55) 15 Lethality.
So we all know that Bonus Arm Pen is items/runes/masteries and Lethality is 60% value up front 40% based on enemy level..
I'm not going to super number crunch this but for a small example lets say an ADC buys
30 armor, and has 9x armor yellows............... 39 Bonus Armor
39(.35) = 13.65 Armor Pen
Flat Armor Penetration = LETHALITY × (0.6 + 0.4 × Target's level ÷ 18) Correct me if i'm wrong, havn't done this formula in awhile, 9 flat upfront pen, and if the target is say level 10, thats roughly 3 more arm pen, total, 12 Armor pen.
Even if they didn't get Tabis, Maybe they get a GA.. Same thing.. What if they have Tabi + GA.. you get 24 Armor Pen from 
Obviously Lethality items offer other things outside of damage, but the anit-healing on
is probably being undervalued for assassins..
Basically what i'm trying to get at is.. Why is Riot making all the lethality items 55 AD as if the 15 Lethality is suppose to make up for the lack of ad.. When Assassins could just keep opting for
and be more durable while having just as much if not more damage.. And if they really needed a Pen item then
is imo a better option to fill that need.
.... Pretty sure the fantasy around "outplaying -/- stat checking" is completely flipped at this point.
/
now.
items. U can build both.
.
: this build delay ur spike and ur "OS" potential
will still be garbage IMO