I've not really tested this, but I was just thinking about it. Wouldn't lethality out damage crit+as on certain adc's, if not all? If you pair it with lethal tempo or hail of blades, that is.
Now since Zeri came out, playing her with eclipse is viable however full lethality Zeri wouldn't really work out. Each of her autos from her Q have their own individual crit rather than burst you can run sudden impact and get it from her regular enhanced auto though. Just not really best with her Q which is meant to be actually used as her auto.
Lethality isn't good on every single ADC. Most will benefit pretty well from it like Jhin, Ezreal or like Kai'Sa, but it's by no means the best option and crit will always be good. It's dependant on when you should build what during the game.
Even if you're fed and going lethality, if the enemy team has tanky units, you're basically useless.
I mean you could add in a Lord Dominik's and seryldas. I also think a hybrid Crit+Lethality build would be fun and may work. With adc's like kog maw or vayne, wouldn't runes + boots be enough as. I guess with vayne it's hard, but kog has his W. But I do agree, it's entirely situational.
I don't see a Vayne ever working well with lethality. The only item she could go is LDR, she does not need a Serylda. It's also not just about attack speed.
The reason you build AS is because the faster you can attack, the more damage you'll put out, thus crit becoming really good. Kogmaw also goes AS -> crit to value damage. Runes and boots may be enough AS, but you're building it for the purpose of dealing more damage. Lethality falls of the longer the game goes on, crit does not.
Even if you're fed and going lethality, if the enemy team has tanky units, you're basically useless.
The reason you build AS is because the faster you can attack, the more damage you'll put out, thus crit becoming really good. Kogmaw also goes AS -> crit to value damage. Runes and boots may be enough AS, but you're building it for the purpose of dealing more damage. Lethality falls of the longer the game goes on, crit does not.