Reasons RIven is balanced?
Because Riven is gated by her lack of innate tank and is heavily punished for mistakes.
If Riven mistimes her shield, you can simply ignore it and wait for it to go on cooldown before harassing or trading with her.
If you've any CC, you can force her to waste part of her damage in order to try and catch up with you to trade back.
Any trade where she doesn't come out with more hp is a lost trade as she has abysmal hp regen.
Riven relies completely upon snowballing, if you can prevent her from doing that or getting successful roams with mia pings, then you can quickly shut her out of the game and prevent her from being a factor later on.