Complaining about how broken an item or rune is something low elo players do. High elo players learn how to counter and itemize against such situations.
If you are up to date with League of Legends preseason, you have heard of First Strike. What you've also heard, though, is the complaints about its true damage. those complaints, however, are misguided due to not knowing how First Strike's true damage works.
First Strike is a Rune that amplifies your damage dealt to enemies by 10% for 3 seconds once you initiate champion combat. If an enemy initiates champion combat with you first you have .25 seconds to engage in combat to activate the rune, otherwise, it goes on its full cooldown.
First Strike's additional 10% bonus damage is based on your damage dealt. Despite being true damage, it is a reflection of the damage you are doing to an enemy, meaning if you do physical damage, the 10% bonus true damage is based on the post mitigated damage you dealt to the enemy. First Strike is mitigated based on the source of the damage. If the source damage is already true damage then First Strike is actually not being mitigated. If the source damage is magical or physical it can be mitigated which also reduces the effectiveness of True Strike.
The problem with making First Strike adaptive damage or just bonus damage is it would mean it gets mitigated twice. The original source of damage and then the First Strike bonus damage would both be mitigated making the rune extremely weak.