I've thought the same thing for a while, .. and to be honest, I still think she has a really odd champion in terms of balance.
As you said, she deals % maximum health true damage. The thing that people are supposed to build in order to mitigate true damage is flat health. So her kit is empowered by the statistic that is used to mitigate the thing that makes her strong?? What???
But, .. I realized, she does have short range for a marksman, low resistances, and no built in sustain. Because of her % max health true damage, she has relatively low Base AD, as well as relatively low base and scaling stats compared to other marksmen.
Also, .. this is the thing that I don't think people think about too much. ... Yes, she is designed to be a tank-buster, but she is one of the few champions who has no magic damage sources in her kit.
What does this mean?
It means her damage is not very effective against enemies with lower max health that build armor.
She is still strong, .. and yeah ... pretty brandead ... but that is her counter.