The "problem" of ADCs buying lethality is a two pronged problem.
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Games are now much shorter than they ever were when crit was the go-to build for ADCs. This change was an intentional move by Riot a season or so ago (exactly when I'm unsure), and they have achieved it. This is the reason we have turrets that can be knocked over with a gentle breeze.
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Crit damage only really becomes effective once you have at least 2 items, and more like 3 to be relevant.
Keeping both in mind, now ADCs have a choice. Do I build lethality and become useful at one item (or even half an item with a serrated dirk first back) and "risk" falling off comparatively to crit IF the game goes over 35-40 minutes, or do I buy crit and KNOW I will be useless until at least 2-3 items?
The current problem is that because of game length, teams get so far behind and lose before crit gets off the ground. Before this recent lethality patch, the ADC in 2k17 meme was strong because there wasn't even this trade off. ADCs built crit and felt horrible. Now a select few who can use lethality are useful now and feel good to play.
In short, lethality itself isn't an inherent problem on ADCs. The lack of a downside to building lethality because crit is weak is.