Her mobility negates all melee champs except super bursters with multiple gapclosers aka 
or 
she is another champ that when released, really has 0 downsides and incredible power. Why does Riot do this?
I'm going to take a guess: you chased her 1v1 as Darius?
Her early game damage is pretty pitiful, and her movement mechanics with her passive are actually REALLY weird for a lot of people to deal with. I personally just can't play her, for instance, but my roommate loves her to death.
Her mobility also doesn't protect her from suppression, knock ups, stuns, snares, roots, or long lasting slows. She lacks any real escape mechanic outside of her kiting potential, making her basically just a slightly more meta Ashe. Her ult requires a large degree of teamwork between her and her Oathsworn (I can't tell you how often I've been suddenly ulted when I was in the middle of being about to pop my ult or some other wide CC spell to lock down a team) .
Yes, she doesn't seem to have a lot of downsides. But early on she needs quite a bit of support to get rolling, her mana costs can hinder her pretty badly through the early game, and it's as easy to mess up one of her most powerful abilities as it is to use it correctly.
She's a champion who requires a lot of skill and finesse to really master, so anyone who does should be rewarded for being able to. In that respect she's just fine.
Pretty much impossible to use efficiently in SoloQ.
With duo bot voice chat it's impossible to be bad at Kal unless you truly suck.
Good - VERY strong with voice communication Bad - Trash SoloQ
