Hey guys, I am going to pop in here and give my two cents on Sejuani.
TBH the worst part of sej (for me) is that she has no feel to her.
Sejuani definitely has a feel to her, and she is really unlike most of the standard champions that every one plays. Sejuani is a really tanky mage with huge burst and amazing crowd control. Sejuani has the ability to do more than 50% health to entire teams, and 1v1 almost any carry late game.
Her kit just feels like garbage.
Her kit is actually what makes her viable throughout the game and allows you to be a tanky burst champion. Sejuania has 2 moves that scale with health keeping you relevant-ish if you build pure tank. And her ap scaling and base damage is pretty good on all of her spells. adding a little bit of ap gives her a ton of damage because of her high base damage on her spells. Not to mention that her passive gives her +25 armor and MR along with 25% slow reduction.
She's like Amumu but worse in almost every way;
Amumu better clear
Amumu better damage
Amumu stronger late game
Amumu better jungle sustain
Amumu higher overall threat
This is just generally not true. The clear times between amumu and sejuani are actually very similar, to the point where there difference is negligible.
In the case of damage, Full ap amumu vs full Ap Sejuani is also similar, therefore negligible. Infact, it probably leans towards sejuani. full tank sej vs full tank amumu leans towards sejuani, but is again negligible.
As far as lat game goes they are both equally useful if played correctly. They both provide similar forms of cc and engage.
Neither amumu nor Sejuani have sustain.
Sejuani has more threat than amumu in most cases in my opinion.
Just going to leave these here.
https://www.youtube.com/watch?v=AxR_tj7ardE
https://www.youtube.com/watch?v=gxfUOz88pGE