Skarner has a pretty linear kit, it makes balancing him pretty hard to do, since his abilities are as reliable as they are and can only really be used one way he doesn't really have opportunities to outplay people, so doing well in a fight depends more on numbers, it's an issue that he's had for a while, even before the season 3 rework, if they buff skarner he dominates without much counter play and if they don't he gets overshadowed, and since the game relies on perfect imbalance since the gameplay isn't symmetrical finding that perfect middle ground for his numbers so that he isn't OP or UP isn't possible, especially since the game isn't static. There are other champions that have this issue as well to varying degrees, all of the juggernauts fell into this category both before the update and now, Aatrox has since release, Quinn since the recent rework, Urgot, Galio, Udyr, Taric, old Ryze. Essentially the more reliable a part of a champions kit, or the kit as a whole, and the less circumstantial each ability is, the more of the champions power budget is used by the ability.
In contrast to this there are champions that have a lot of options in their kit for how each ability can be used, so using them correctly is important, or lack the same reliability in their abilities, Ahri for example has to land skill shots and needs to angle Q to hit on the return in order to deal full damage. She also has several build options depending on the situation and has quite a bit of nuance within each ability, allowing her to adapt. Aside from a few times where she was a bit overpowered Ahri hasn't really had the same issues as other champions, it's either a really good idea to pick Ahri because she's really strong, or it's a reasonable option because she is relatively balanced, and can still make a positive contribution to the game if she goes even or falls slightly behind. The fact that she can fail her abilities or the enemy can dodge them means that when she is being balanced, her power budget doesn't need to exactly match the overall state of the game, as long as her worst case(miss everything) is weaker than the baseline, and her best case(land everything) is above the baseline by a similar amount, she will not cause problems for the overall health of the game, while still being easily playable. Orianna is the same way, rarely overpowered, but always viable.