lethality gives 1 armor pen vs targets level 18, so its ALWAYS worse vs targets lower than 18, given the same amount.
it is equal vs targets that are level 18. So best case scenario it is equal, levels 1-17 it is worse.
BUT incredibly inefficient vs tanks, and realistically even useless vs very high armored targets. 250 armor gives 71% reduction, and 200 gives 67% so even if you had 50 armor pen, you would only do 4% more damage.
the difference between 150 and 100 is 12%, and the difference between 50 and 0 is 33%.
thus, the higher their armor, the more useless flat penetration becomes.