Sabbath is not a hard condition for MO IMHO, as far as I have faced them. The only issue is that the opponent should have a 4P card on their side which should be possible most times, but in a short round, I agree Yaga is a 1 for 13 and Filavandrel is a 12P Sorceress.But Yaga is basically a Master of Puppets with less base strength, more provisions and which is worth 1 for 13 without Sabbath.
So, I can see fierce competition between Filavandrel and Yaga to be the "Worst Legendary from Pop". At least you can use Yaga in R1 and R2 to threaten and bully the opponent out of the round. Filavandrel on the other hand is just bad and bland.