Sejuani really only has one problem right now
I hope they address this in the update but her biggest problem is that you get counterjungled incredibly hard. If you want Sejuani and pick jungle first the enemy can counterpick graves, Lee Sin, Pantheon or whoever and just blow you up on either your first or second clear. This is probably why Sejuani is the least playable tank in the top lane. She has such a horrible ability to defend or attack that her only chance to thrive in a game is through praying the enemy jungler doesn't know how to play the matchup and let's her afk farm for 10 minutes.