not really, flanking is the most effective tactic of getting behind people, you cannot counter it unless you are running and checking your back at the same time. If you are in front of their eyes, you might as well shoot them, not strafe backstab.
Also, you have to hold the D pad ability slot for ~5s to switch to an ability (assuming it isn't passive), so I doubt anything will be OP.
Anything that is OP will be among the first few unlocks. They might take an ability out if it isn't fun. Cutting an ability from the game is the most extreme measure they can take with balancing, otherwise they'll just nerf it.
Just because you obviously didn't read my previous comment that I was referring to....
As always, there will be players that are awesome with the OHK backstab.... but they could always put a cooldown on that to so the monkey operatives can't go flying around the map spamming the backstab. And yes, I've seen people who have knifing godliness, so I know it's entirely possible.....
OP=over powered..... Let me explain, Operative backstabs guy, operative does flippy thingy and backstabs another, then immediately another, then run-slides behind the guy shooting at him and stabs him too, 3 seconds have passed. Obviously if that was possible, in the right hands, the backstab ability would be very powerful, possibly even OVERPOWERED (OP*)..... Stabbing people in the back because they're too [censored] to check their "corners" as they run by is just funny and that idiot got exactly what he deserved. Jinking, Juking, Jumping, and Diving all while backstabbing an entire team that is shooting at you would be a little ridiculous. Don't say it won't EVER happen either, there will be some epic player that will pull it off on a newb team.... UNLESS, there is a small but slight cooldown for the ability.
Just enough so it can't be constantly spammed as someone is trying to run around and get behind their enemy.... or even better, put an animation on it.... then you achieve 2 things, the guy getting stabbed realizes just how epic you are and how [censored] he is for not checking his back, and 2, you are frozen and free to be shot while you are putting someone else out of their misery. That would make all who use the ability seriously consider their actions before doing them. It would also ensure that no one would ever be able to call the ability OP (overpowered if you're short attention span forgot already) because in the process of OHK someone, you are making yourself extremely vulnerable.
*edit/add*
Well you forgot they have a shield ability allowing them to take damage for other players, have multiple types of grendaes, plant explosive charges to open up new routes to objectives ect. They have released as much info on soldiers as they have on all the other classes. Although I will say it seems they have released the most information for Operatives telling us about interrogation ability,hacking,backstab,sticky grenades, playing dead as well as being able to disguise themselves.
Lets not forget though that we are apparently going to be limited to the number of abilities we will be able to use. So even though an operative might have all those abilities, they might not have complete access to them at all times. Granted, the amount of abilities a character can have is just what I've read through others posts, I haven't heard/read any developers blogs saying that.