Balancing 'Difficult' Champions

This is partially inspired by the recent threads on 's somewhat questionable state of balance, but I think it's an interesting topic regardless.

I think most people would agree that margin of error is relevant in champion balance. Champions like or have access to incredibly powerful, long range hard CC primarily balanced by being skill shots - meanwhile, champs with point-and-click hard CC like suffer from limitations such as short range, low duration or obvious tells.

So, what I'm wondering is this - how far can this go? Is it justifiable to have champs like with absurdly high damage limited by delayed timing and mechanical difficulty? Or alternatively, what about champs like who have access to incredibly powerful displacement effects conditional on very precise positioning and good reaction time? Hypothetically speaking, how powerful does a champion have to be before no degree of mechanical difficulty can vindicate them?