You are wrong for the following reason if i had 1000 health and took 20% max health damage, i would have 800 health If i had 2000 and took the same damage i would have 1600. Because you don't only take percent health damage, you will be tankier if you bought health even if you take more damage
No, they aren't really wrong.
Let's see two possibilities here:
2 000 health, 0 armor = 2 000 effective physical health 1 000 health, 100 armor = 2 000 effective physical health
After taking 20% of max health damage as physical damage you are left with:
1 600 health, 0 armor = 1 600 effective health 900 health, 100 armor = 1 800 effective health
It's about getting only some health, while getting the defensive type that will mitigate the %health damage.
However, it doesn't mean you should negate health effectively, as there isn't even a single champion that relies only on %health damage. You should just avoid stacking health without getting other defensive items.
and then they pick vayne and have a bork as first item and you realize you life as anyone who isn't an assassin is meaningless.
The reason you shouldn't is not that it's ineffective, rather that it is less effective, unless you already have plenty of Resistance.
, against aa type like Vayne that is.
You shouldn't buy just health, though. You need other defenses to mitigate some of that damage, which makes more health more effective.
There may be one reason to avoid health against %health damage: Lifesteal. You are left with more health, but they will also heal more.
But I don't think that it would actually screw things in their favour.
