Riven difficulty so high.

She requires pretty solid awareness in teamfights to maximize her damage output. Also, the fact that she literally has the worst sustain in the game makes falling behind very punishing.

Also Singed

I play her top often, I can definitely vouch for difficulty using her. She can still destroy, but falling behind by even one level is crippling. You'll need a good jungler if that happens -you also need the feed from ganks. She's not terribly efficient. The steamroll you can get later though is worth the hassle if you can manage it though. Just practice with her a lot and you'll have less trouble in-game. You just need to play her consistently for awhile to get used to her. She's not a pick up and pwn champion. Knowing when to E away or E in or when to W or how to divide your Q's, how you combo, when you go in, when you ult, the wall jump timing, it all takes practice.

Oh, and know that your health isn't everything; you have a shield and two stuns. If you can burst them in one combo and your health is low, go for it -as long as their counter ability (if they one) is on CDR. That was my biggest mistake starting out. I played too safe and got poked out of farm which lost me lanes. Farm until you see openings and floor the gas pedal when you see it. I recommend saving E for escapes early on since your damage isn't high enough to insta-burst them; it makes for decent poking.