Yes, she IS strong. Her whole kit is about hopping around while dealing tons of damage. Her damage is mainly physical, so an early chain vest into SFC/FH will make laning against her a breeze. Don't fight her or hang around at low life, just play as you would play versus Garen. If she uses her ult, retreat and come back when it wore off, also don't initiate when you don't know for sure you can fight her if she has her ult ready. If you can avoid playing against her with a melee champ, you have an huge advantage, as you can poke her down and she has nothing to bring up against it but her shield, which is on a fairly long cooldown and kinda weak as long she hasn't tons of AD or she isn't maxing it 1st, what Rivens mostly avoid, because it lowers their snowball potential by a huge chunk. This is what you want too, fight her before lvl 3 when she is strongest, ask for an early gank, if you can kill her early or deny her farm she will probaby buy tankier items on her 1st back, what decreases her farming potential by a sh*tload. Generally, handle her as you would handle Nasus, don't feed her early on and don't give her free farm or she wrecks you lategame.
Also note that her Ult increases her spell radius, what is often underestimated, her q hops will get a bigger aoe and she can chase you with ease, her shield-dash takes her further and her w radius increases.
In Teamfights, if she builds damage focus her right after the carries, or cc her as she gets wrecked by cc more than almost any other champ. Hard CC works well, like silence, as she relies on her spells to do anything, and gets totally useless if she can't use them.
If she builds tanky she will try to interrupt your team with her stun and knock-up, but you can usually ignore her if you aren't on critical low health.