That's what people who dont know how to play her say, i mained her so let me give you some tips,

Let me explain: She is at her prime during mid game, that is when ADC and putting maximum effort into agility and junglers too and mid laners into intellect and supports, this is when they are at their weakest and you are at your strongest due to assassin/caster prime cards, once they finish that they move viability where you now lost your burst cast damage and you will NOT be able to finish them with your combo because of their increases investment into defense (Health, armors and shields ect), you NEED to put the enemy team behind before this happens.

When you play her you have a small window of opportunity to win and weaken the team in terms of CP and XP to help your team adc jungler and others gain an advantage indirectly,

First, shes a burst assassin, our goal is to take out other casters and ADC's and low vitality build supports,

SHE IS NOT LIKE OTHER ASSASSINS She needs to be played differently in the sense other assassins like Kallari keep on getting better and better as once they finish agility they can increase their durability with vitality, you cannot do this with countess as you are an intellect hero and a caster assassin who invest firsts into intellect and then vitality and then agility so your base damage is very low.

Watch streams to understand what i mean will explain more later