I don't think we have an issue with ADCs building assassin items until lethality was introduced. MF had a Korean built with Youmuus and Black cleaver before, but it wasn't as bad as she still build standard items after. Now she can build all three assassin items plus blackcleaver and she became an absolute one shotting monster. 60 flat armor pen + 45% armor pen means there is literally nothing left to defend yourself.
I'm not gonna say that I don't enjoy one shotting people, but for ADCs to abuse lethality from ranged is unhealthy. It's unfair and unfun for everyone (except for my team cuz freelo).