Well first, yes they do have weak early games, in fiora's case pick renekton into her you'll be perfectly fine and likely snowball off of her. But the reason Percent max HP damage of any kind is good is simple, they wanted to avoid giving champions burst, in Kled's case if his Ultimate didn't do percent max HP damage and did flat damage instead he could dive your back line and kill you instantly. If it were current health damage it would be useless, in fiora's case and vayne's case they needed tank buster's, some tanks become unkillable at certain parts of the game but do it to a point of, well ACTUALLY being unkillable and unhealthy. So the best way to counter that is to put a carry in each role that can counter them, vel'koz, fiora and vayne are riots answer to unkillable tanks, counter pick them. And while yes true damage is annoying it's a necessary evil in many cases, to be honest i'd rather fiora do a miniscule amount of extra damage from hitting a very easily telegraphed ability on an enemy than to do flat damage and be an early game monster.