Any sustain - while Shyvanna doesn't need mana, she has absolutely no health sustain or way to avoid damage.
Ranged harass, she doesn't have a proper fast gap closer except for her ult, her skillshot on itself is negligible even if it hits (It shouldn't).
In fact, disregarding player skill and external factors like ganks, Shyvanna is outlaned by most solo lane champions.
As to the champions in your post: Nidalee should be fine, she can poke all day and heal whatever Shyvanna throws at her.
Riven should be more than fine, she can easily trade damage with Shyvanna. Her W, knockback on Q and shield on E should make it next to impossible to do a bad exchange. You might want to play it safe till you've got all three of your skills though. After that, the standard harass should be fine (Q-Q towards her, W, autoattack, Q for knockback, E out.)