I've been using the Planet-X brakes for two seasons now.
No cable slippage.....but it does LOOK like something that one would be concerned about, so I make sure to use blue loctite on the little set screw in the ferrule and make sure it is snugged up.
I remove my front wheel every day for transport, and so I pull the ferrule out of its seat so I can get the tire past the brake shoes. I haven't had any problem with the ferrule loosening.
As far as braking power goes: I am in Utah, and do a lot of high speed mountain descents. I have had no problems, but I admit that I don't seem to be very critical of brakes. I wnt about 10 years on one pair of brake shoes
I guess I just don't descend that aggressively, but I have never been dropped on a descent. I use the brakes more like "speed adjusters" than "panic stoppers".