Jaina's power was built over the years and we pretty much knew that she had the power to do what she did. Sylvannas on the other hand has no build up in terms of power and whatever happens will be out of Danusers butt. Even with that beating an old orc and a pathetic version of the Lich King doesn't make her that powerful. If the other heroes weren't occupied with Anduin during the fight she would have lost.
Death Knight Anduin however. Now that is a powerhouse that puts Sylvannas to shame. Charged straight to Kyrestia one of the eternal ones and got her down with one strike and not to mention killed her council around her with just one move. It's as I said. If the Jailer can turn Sylvannas into something relevant then what would happen if he gave that power to Anduin who was already a powerhouse.