You have to remember that assassins always need to stay ahead of or with the curve of power to be useful. While the build does damage, if she is behind the power curve, it just isn't going to be enough.
Tanks, supports and bruisers have enough base utility to be useful if they are behind and build paths to make up for lack of gold. Assassins have no such luxury. She must build damage to utilize her kit. She just doesn't have the base numbers to really be that powerful compared to other roles in the jungle when behind.
This is true for pretty much all assassins though.