Riven is not overpowered or balanced she is just strong
(If you're someone you hates Riven and think she's overpower I'd advise you to keep my points in mind)
Here I'm about to counter most of the arguments stating why Riven is OP
1.Riven is too easy to play for how strong she is
Riven is easy to play I'll admit that but just like many other champs it's easy to tell the difference from a good Riven player and a bad one.A bad Riven player spams all of their abilities not using their passive correctly and uses their skills in no particular order.A good Riven handles their abilities better for example, maxing E before Q against heavy poke enemies abusing their passive to its full ability by quickly canceling the autoattack animations and they know burst combo's that cancel ability animations and dish out damage quickly.When Riven gets behind its hard for her to come back unless you try and gank another lane or farm like crazy. Bruisers rape Riven
(yes Riven counters Nasus early but after he gets about 200-300 stacks and a frozen heart Riven cant handle him anymore)
2.Riven does so much early game damage
Yes she does Riven is a mobile champ thats supposed to punish you when you get too close she is a supreme lane bully just like
and a few others thats the trade off here is that Riven has no sustain her health regen was nerfed her health regen now is (3.3) when you do damage to Riven it usually sticks and its hard for her to heal its back up thats why its great to play champs with alot of sustain

3.Riven is ungankable Riven is hard to gank but its not impossible you gank her just like you would any other mobile character, wait for them to use their mobility move(s) Leblanc: Bait out W then gank, Zed: Bait out W then gank, Fizz: Bait out E then gank. You do the same for Riven and if your jungler comes to gank just bait her by trying to farm safely by a distance and she will use her dashes E-Q-Q-Q- and W for her harrass then your jungler ganks and she cant escape her whole kit is on a cooldown and at early levels her cooldowns are high. After Riven gets her cdr items she is incredibly mobile and can chase or run away excessively but Riven gives up being tanky and is most likey building glass cannon point and click cc is effective against bringing her down once Riven gets cc'ed she instantly gets melted due to her lack of defenses and health.
4.Riven's animation canceling is OP and provides no counterplay Yes this is true a good Riven can unload a combo on you and can kill you within less than 3 seconds but this usually happens only when Riven is fed or at full build also these animation cancels are extremely hard to learn can take about a week to learn how to do it and a month is master it and be consistent with the combo and the slightest mistake gone wrong in this combo you can get away or kill Riven if its a teamfight and riven frucks up she is cc'ed and instantly dead leaving your team in a 4v5 just attempting to make the play. here are some animations cancels for Riven that she can perform starting from easy to hard: https://www.youtube.com/watch?v=uvIJBvNBZkw
5.AD scaling shield is OP Not much I have to say about this her shield just rewards her for building damage which again she trades for tankyness because when she builds glass cannon she dies too easily and since Riven is a fighter her helps her stay in the fight a little longer at about level 18 Riven only has about 2020(varies depending on build) and thats not terrible for a fighter
Riven is very annoying to play against and she is an easy champ to play but she has clear downsides and weaknesses. Riven isn't ovepowered or balanced just strong.