Lethality is useless and is not rewarding
It is more effect on marksmen who buy the items that are meant for assassins. IMO it is just terrible because even when you are ahead as an assassin you are punished because of lethality and you still can't do anything even if you are ahead. You never get rewarded for being ahead. It is just "Oh you are 4-0 sorry the ADC is two levels lower you can't do any damage to bypass their armor haha sorry." It is almost like lethality just punishes you for being ahead at all.