remmember she slows and because you are full tank and no dmg, the enemy just has to turn arround and focus the other 4 and due to the enemy being more stronger in terms of dmg, the 4 are dead , sejuani will be the one left and after some time will go down.
Althought sejuani has her pros which are the chase ability, but in team figh she wont be what other champions are.