A good Riven would make you believe that. If you don't play riven, but found her easy because you had success when playing her in a few games, you would believe that. She's really good at dueling, and killing 1 or two targets quickly. She can kill like an assassin, but once she goes in, she can't leave as easily as zed, Lb, kat or become untargetable like fizz, yi, elise. If she used her q's then maybe she can dash away, but the dash isn't that long and it doesn't go through walls. She builds damage and deals damage, as what you would expect. She doesn't build tank and still do damage, so if she's behind it's very hard to come back. If she uses her mobility abilities to catch you, she has nothing to fight with. And she pretty much has to run straight towards you to get to you, as in no dashing in like irelia, wu, xin. So if you can cc her or slow her before she can reach you, then she will do incredibly less damage. It's not like her ult does true damage and resets on an execution >.>