The problem imo is true damage, in theory it's great, tanks build resistance thus true damage. However you're left with no way to counter it, thus the only people resistant to that damage are health tanks, so in making a tank assassin you've simply made an assassin who can kill tanks.
If there were a way to say, if enemy target has +3000 health they receive the damage as true damage and if they have less they receive it as magic damage it would be ok (though convoluted).
That said though aren't carries/mages both counters to tanks? tanks don't have the damage to outright kill carriers shy of attrition and typically don't have a way to fight against kiting. Amumu sure, but a simple side step solves that problem, alistar not really since he still has to cover the same distance (unless he hits you into a wall). Shen's taunt has good range however Ashe's arrow has more. Etc Etc.