League was too immature in Season 1 to even care about that broken shit and Black Cleavers was implemented during preseason testing. Broken shit was expected. I would rate Black Cleavers higher simply because it wasn't champ specific.
But back to Sejuani, I honestly don't see her being too broken. She is definitely strong but the nerfs on PBE will only give a slight hit (imo), meaning Riot doesn't consider her too out of wack atm.
Nightblue3 claims, and I agree, that Fizz is the most OP jungler atm.
The only problem Sejuani has is that she can be outscaled and is not technically a "carry" jungler. I've had plenty of Sej games where I'm double-digit kills and can soak up enough damage for my derp team to pull a win, but sometimes you get that freak game where even being fed can't make you carry hard enough. I had a Sej game yesterday where all 3 lanes "lost" but I went 9-6-15 and had a full 6 item build. Literally couldn't carry a win even though it would have been so easy if the team just grouped around me.
Fizz on the otherhand is a carry jungler that has insane damage AND builds tanky. You won't get outscaled. 6 item fizz can 1v1 anyone. If you go late you can win games without needing your team to group with you.
Since the Cinderhulk update I'm 8-5 with Sej. (62%)
Since Fizz became a meta jungler I'm 21-11 with him. (66%)