True, %, pen, and shred dmg would be OK, but those with it do tons of normal dmg and heal/shield

These stats exist to make a champion better able to kill tanks without making them overkill squishies. The thing that makes a lot of the champions that have this shit frustrating is that they often are able to do so much regular damage that they still manage to overkill squishies if they get even slightly ahead or are allowed to scale. People understand that Master Yi, Fiora, and Urgot with Cleaver can melt a tank, but it doesn't seem fair that they are simultaneously able to do that AND kill your carries just as well as any other assassin or bruiser can.

You buy a lot of damage? They nuke you, then heal off what damage you managed to do to them, staying in the fight. You buy a lot of defense? They shred you and you do so little damage that they outheal it.

• Sustain
• high damage
• Strong anti-defense stats

These three things are all okay by themselves, but when you give a champion easy access to all three of them, it creates something that feels extremely frustrating to deal with when it is strong. A large majority of the champions that are the most complained about tend to be good at all three. Remember back when Vayne was doing tumble crits for 75% of the other ADC's health bar? It wasn't just bullshit that she had % HP true damage, it was bullshit because she had it while also bursting carries and drain tanking with BotRK later on if you didn't chain-CC and focus her before she could get more than a couple shots off. The current worst example is Riven with Death's Dance and Conqueror. She deals tons of flat damage while also healing tons of fucking damage and melting your tanks. When a champion is like this, the counterplay is often basically "don't even let them play League of Legends" because you lose if you let them interact with you in any interesting way, which is not fun for either side.