Well, if Riven dashys-dashys-dashys than she has no damage other than autos and a stun that STUNS HERSELF TOO. You're seeing it from the wrong side.
She's supposed to be a mobile snowball, high-risk/high-reward bruiser type champion much like Irelia/Fiora. If she falls behind he does nothing. In the contrary I've seen 0/5 Irelias and Fioras 2v1ing.
Not having mana is good but at the same time its a double edged sword, it makes having items that other bruisers ABUSE a complete waist on Riven "cough" Trinity Force "cough". Plus she's HEAVILY RELIANT on her abilities if she had mana she would actually be the worst champion ever designed.
She provides 0 magic damage and most of her damage comes from autos, build tabis and a tank armor item and she cant touch you anymore. Unless you're dumb and allow her to.
She was released as a lane bully snowballer, then she was nerfed into a get bullied in lane but you still have to snowball or else you're useless and now shes either a shove lane roam glass cannon assassin wannabe or a don't die, farm and let me engage and hope my team follows.
She can use pretty much use every rune but none of them scream "USE ME, YOU NEED ME", spellbook is good on her since flash is important to always have on her, but you could say that to about every champion. Conqueror is good but it feels like it restrains your trades to a predictable pattern, or else you don't get the full value out of it.
Im not even gonna talk about how strong Fiora, Renek, Irelia and Urgot are right now. God, even the easiest matchups ever for Riven, like GP and Teemo have become way harder.
Her build path got cucked since you now build full HP to get BC(I don't understand this lmao). You basically have to get a warhammer first and then get BC(basically you delay your first item by 1100 gold...) while every other top laner can just rush an item due to their high enough base damage.
She requires low ping, heavy combo (just gonna leave this here for you: https://youtu.be/E0h0UqkvYqc) and matchup knowledge.
She can't be played jungle because even tho she has a decent clear if you know how to combo, her ganking is lackluster. She can't be support because, well you might as well play pyke or leona if you want strong all in supports. She can't play mid since other assassins assassinate and roam better than you, mage midlaners waveclear better and safer than you. This leaves you top, but what can you offer top? You can't tank, there are way better splitpushers, duelers and engage.
I've been playing Riven for about 3 years now and one thing i can tell you is, I don't want her to be strong, one of the reasons i took time to learn her and play her is the difficulty, the effort i have to put into her, but I'd like her to AT LEAST be decent, and seriously, right now she's below that.