The Atlus, Banshee, Phantom and Brute can all kill you in one hit if you are close, even at full shields and health. I have not observed such a from the Geth. Can the Pyro or Prime or any other unit kill you in melee with a single attack?
The Geth do not have a unit that can insta kill. However, this is balanced by the multiple units that have high burst damage.
Also, the banshee's melee animation is quite long and can be interrupted if a biotic explosion is triggered before the impale occurs or if you had a missile launcher ready to take her out (the animation to bring out the missile launcher and to fire takes too long for you to save the party member).