It absolutely ruins teams who stack the Heal summoner spell. (which is good, because frankly, 3 players using Heal at the same time is a massive HP swing in teamfights, far more powerful than any other summoner skills can account for. I'm not sure why LoL hasn't learned from Mekanism abuse in DotA).
It quite simply, does not "ruin teams" who stack heal. On a single target, if you have managed to hit them in the last 5 seconds (so in pretty much every situation this is going to be just 1 champion) gets 40% less healed. If the champion you have debuffed is level 18, they are going to still restore 300 health (total heal is 500, 40% off is 200). If the champion you have debuffed is not the one themselves to use heal (an ally uses it nearby) they will be healed 150 instead of 250. Even if everyone on their team uses heal, its really only going to prevent a potential 600 healing, assuming that every champion is level 18, and all in melee range, which is pretty much never. Now if everyone on your team got this item, and attacked seperate targets, that'd be great, but if your all attacking different targets then i doubt they are going to need every single one of those heals to beat your team.
Considering this is a mid tier item, its not really viable in its role its ment for. It doesnt have good enough base stats to be useful for the price, considering that even if the other team has a lot of heals/soraka/taric that particular debuff wont be helpful against every target. If this combined into a late game item, then it might see some more play, or if its combine price was greatly reduced, but currently its a waste of gold for the minimal benefit given