Riven is actually a good example of one of the glaring problems with the "Fighter" class in LoL

It's not even about her mobility or her abilities that makes Riven problematic. It's the fact that unlike Fighters who typically have to build a wide array of stats to remain viable in a game such as (The last two at least require some core items before even being relevant), she only relies on one statistic to remain a threat.

Attack Damage.

Everything Riven has scales off of her Attack Damage stat, to include her shield from her E. She gets survivability for building offensively. She gets more damage on all of her abilities because of only having to really pump into one statistic as a vast majority rather than having to buff her armor and MR to obscene levels outside of or the occasional . She doesn't have the same level of item dependancy as other fighters, and for lack of a better example outside of the God of Right Clicks and RNG himself she basically represents everything a fighter should be. Somebody who can get into the brawl and stick into it while not melting as immediately as an Assassin or Marksman caught in the crossfire. Part of the reason why melee carries and beefy fighters like Spirit Breaker and Chaos Knight work in Dota (as an example, of course) is because they don't have to build the same wide array of stats that every Fighter has to deal with in League.

That and Riven's resourceless, but that's an entirely different can of worms.