People of boards, there's a reason why Riven has been complained for years
People hate her for a reason and have been complaining for years, about since her hotfix after release.
She has ridiculous damage and incredibly high mobility, the wall jumping wasn't intended until Riot added it.
She scales so ludicrously well with cdr it's disgusting, she can have her Q up permanently and spam her AD scaling shield every 4 seconds(it lasts 1.5).
She has 2 forms of hard CC in her kit.
She has a Bruiser kit but able to dish out assassin level damage within 2-3 seconds giving little counterplay to the enemy unless they have hard CC to stop her, high mobility, or both.
Her animation cancelling is disgusting in lane, she can just combo you then dash out if she wants to, and if you do get back in range as her she can just stun you or decide you aren't worth living in her lane anymore.
And the whole "She has to snowball or she's worthless" is the worst possible excuse I have heard, she doesn't need to snowball, a Riven who is behind to say an enemy Darius can just pop her ult for a huge AD steroid and have her passive enhance her autos immensely.
The fact that she can spam her spells in lane all day while being resourceless is also annoying, but that is a huge issue with most resourceless champions unlike Garen.
The worst thing about laning against her is how long she can keep her Q up to zone and the cooldown only starts at the first Q, not last Q.
Where's the fair treatment to Garen?
She isn't hard to play by any means, the animation cancelling is maybe slightly difficult to get down, but that doesn't warrant the insane damage.
People have been complaining for years and simply telling them to "git gud" isn't going to cut it, there are high tier players who say she is toxic as hell and unfun to play against or just obnoxious in general.