The problem is that facing is pretty random in FE. I think its better to just do general purpose abilities or maybe to do a flanking ability where you just need one other unit next to the target. That is what they did in D&D 4th edition because backstabbing is pretty complex for a simple damage bonus.

If they wanted to make it a really cool ability they could give the assassin a teleport strike where they appear behind the enemy unit relative to your starting position and then make a normal attack. You could give a bonus like +20% crit chance as well. Since this is strong it should probably be once per combat. 

Well designed abilities like this would make tactical combat way more fun.