Dark Harvest is a stacking mechanic that rewards champions for making big risk plays, mainly assassins. If you can not cooperate with your team to be aware of the position of a champion with dark harvest then thats on you. Being able to play around where an assassin is located is the point of the game. Riot has also recently blessed ADC's with a new cheap active, Stopwatch. Its 600 gold and gives you a invulnerable, untargetable stasis just like Zhonyas. I suggest including this item into your kit when possible. It's really satisfying seeing an assassin over-commit to a kill just to see you Zhonyas and then your team tear him apart.
Also if Riot could raise the PBE sign up requirement to Honor 5 and be at least Gold they could receive more effective feedback. Or just make it like a seasonal reward.
If anything in this game is broken its probably Lethal Tempo. A Kog'Maw with Rageblade, IE, BorK and Runaans is the single most disgusting thing I have ever seen. The game is pretty balanced for the meanwhile, Good job Riot.