The problem with old Sej. The problem with current Sej.
Old Sej: Hard scaling super tank with incredible late game numbers and barely anything someone can play around besides her missing her ult. Her ult was a projectile Amumu ult that full-on stunned people, and the rest of her kit was a mish-mash of Gragas and Amumu linked together with a defensive steroid. Very little skill expression and and very little room to give her a niche. She was just "da tank", and she got incredibly strong as time marched on in a match, so Riot gut the hell out of her early game, which sucks because junglers have a job in the early game that she wasn't able to do anymore (We can say this exact same thing about Sona but I'm not gonna be the one to do that anymore. Still salty, though).
Current Sej: Riot decided to remove Sejuani's aoe ult, but re-gift her with more chances to proc hard cc through her basic kit. All fine and fair. Her new W is now a skill shot to aim that helps stack her E quickly when you land both hits. Alrighty, nice. She gets a new passive that makes her significantly more tanky when making initial engages, seeing how she's charging in head-first (or the boar's head, but whatever), but drops off to poke and stray shots if you have that on your team. Cool beans.
Here's where the problems arise: Sejuani's historically been tooled for late-game team fighting, and now Riot just up and says "Nah we're moving her from Cho'Gath and Zac's space to something more like Rammus's space, where she's good in the earlier stages of the game". However, they didn't necessarily compensate any of her nerfs with mid-game power. They've simply been blanket nerfing her. If they don't want her late game 5v5 to be as strong, that's cool, but that means things like ganks, clears, and her mid game strengths need to be targeted for compensation. Since Sej has a lot more single-target emphasis now, it shouldn't be hard for Riot to just up and do this soon, but they'll probably wait to let her cool off in the competitive space before trying, first.
Long story short: New Sej is apparently meant to excel in mid game skirmishes and picks in comparison to other tanky junglers, but Riot did only half the job of putting her in that space.
And if Big Lincoln sees this: Yes, most of this thread was thought up after I nearly barfed from what you linked me in Discord yesterday. No, reverting Sej is a terrible idea, and she isn't, nor was she ever, a diver champion.
[slayer-jinx-unamused]