She's annoyingly powerful if you don't know how to deal with her. She has a lot of tools at her disposal that make her dangerous at all points of the game. Wave clear, easymode poke, good scaling, zoning tools, disengage tools, she has a lot with little actual skill required. When you really look at it, her most technically challenging spell is her wall, and you can still accomplish a lot without it. Her Q is basically an aoe morgana Q that ignores front line targets and stuns. Her E is something like a 1:1 ap ratio low cd single target nuke that only requires the enemy to be chilled for max damage. And her ult is just a circle of slows and death.
Nothing really challenging by any means, it's just the range on most of her kit requires her to be good at positioning. And most people are ass at positioning. If you can do it though, mana issues are non-existent now because of the infinite mana build of roa, chalice, tear, which are conveniently all viable anivia core items. The skill cap is basically limited by your ability to position yourself.