Shes never the best exactly, but playing her requires alot of intricate experience. For starters, make wide turns around peoples front since as you said, the cloak is really frikkin weak. Move in really fast for the backstab, and after its over, if they live, hit RB to sword lunge them down, or just try to run (the second option rarely works against gunners, tanks, or assaults cus of charges and whirlwinds). The big thing is using dash really well. Also, dont forget about the smoke bomb, the blind helps a bit, but the big thing is if you can time your jump properly after dropping it, she does a big leap way up in the air that you can use to cross like half the map, learn where you are and use that to completely exit the area.
Also, I found alot of luck using the golden armor endorsement, as it gives you enough health to survive getting in the heat long enough to stab someone and run.
She's just an advanced class to play, meaning you either need to use her alot and be creative, or just wait until your really good at the game to have any real luck.
I recommend the first. I also like using her to hunt down annoying snipers who try to camp their bases.
Edit: Back when I played I managed to do matches with like 30 kills and 1 death because I got so good at getting behind them, murdering them and smoke bombing out that I just chain murdered people the entire match. The point is she can own just as much face as any of the other classes (and they all can do some horrible things if played right) once you figure her out. Also, this may just be from a lack of experience with it, but I feel like it would be hard to pull off her extremely fast and sharp movement well on a computer, the xbox controller seems much more fit for it. Im sure long time PC gamers find a way to do it though, since they don't seem to struggle with any other games, so its probly just an issue of preference.
Edit..Edit: I forgot, this is a big one. Your cloak makes a loud noise when its active, which causes people to use their slams or spin around when you get close, you can actually pull off a good backstab by moving close to them cloaked, then dropping it at a certain range and just stabbing them while visible, keep in mind you can also (last I played on the 360 a few months ago at least) cloak while doing the grapple, in order to make you harder to hit while your doing it. This may have been changed. The important part is to drop the cloak before you get close, they never see you coming.
Don't forget to use your lunge atack on stubborn gunners and tanks who dont die from the behind grapple, it almost always finishes the job if your fast enough.
