Dark Harvest in ARAM is overbearing
Over the past few weeks I've noticed Dark Harvest being extremely overpowered in ARAM. Soul generation is just way too much with how many deaths happen in ARAM and since teamfights are always happening you are basically guaranteed to get it stacked by 15 minutes, if not much earlier. In Summoner's Rift it's balanced because you actually have to play around your stacks and collect them from minion waves and it takes a longer period of time to come online. I've noticed every time I take Comet/PTA/Aftershock/Electrocute, I do noticeably less damage overall than I do with Dark Harvest. There is little gameplay interaction with the souls in ARAM compared to other Keystones, there is basically no risk to having it with huge reward, and there is no reason I should have 150 souls (fully stacked Dark Harvest) at 7 minutes as Yorick and do 4500 damage with the keystone at 15 minutes.
As for suggestions on fixing the problem, I would suggest changing the amount of soul essence champions drop from 5 to 3/4 so the keystone is still viable but it takes longer for the damage to ramp up and teams not being able to snowball as easily.