I like how they are not in, but its a shame. It could have been cool if say, backstab only worked from behind and from disguise. If you are not disguised, simple no backstab for you my friend! And it could have been a level 5 ability "Backstab - Ability to use your knife in close proximity to a players back for increased damage" Will still remove your disguise but if used correctly could take a whole team easily.
Then again i guess i just argued myself out of liking the ability
"Kill a whole team easily" No thanks? sounds op to me.