A disclaimer before we get into this: I mostly play Sion and Poppy, so I get more out of Courage of the Colossus than most players. That means you know if I want it nerfed, it’s probably a problem.
Another disclaimer: I like the idea of Courage of the Colossus. Giving another shield to tanks who want to dive in and start fights makes the diving more fun and more rewarding for tank players. HOWEVER, in its current state, Courage of the Colossus is incredibly unhealthy and in my opinion one of the main reasons (if not the main reason) AD Carries are hurting right now.
Yes, I hear the complaints about assassins and burst mages one-shotting AD Carries. To me, that’s part of what you sign up for as an AD Carry — I feel very bad for you, but your survival in those situations should depend on your positioning and the ability of your teammates (like me, your friendly tank top laner) to protect you.
What you didn’t sign up for, however, is a Poppy charging at you under your tower and taking zero damage while killing you. All without any damage items. That’s in part because Poppy is ridiculously strong right now...
But it’s also because Courage of the Colossus is a problem. It’s become a staple for pretty much all tanks, no matter which position
Courage has been nerfed before, but that was just the numbers (cooldown and max health). In order to fix this keystone, there needs to be a larger change.
An easy fix that will go a long way, in my opinion -- scale the shield with how many enemies you crowd control. Meaning if you only CC one enemy champion (like in lane), you get a smaller shield than a 3-man Sion knockup. That still encourages big engages in fights while removing a lot of the extraneous power in other situations.