Disc brakes can be shockingly poor, but here is a little anecdote about an experience I had.
One of the kids bikes was stolen a year or two back. When it went it had excellent Shimano Deore M615 hydraulic brakes, they really did perform as well as my XT brakes.
When we recovered the bike the little scrotes had stripped the brakes and installed (badly) some Tektro cable operated shoot (Novela I think). They had also swapped the good wheels for junk so we now had generic unbranded rotors in place of some end-of-line discounted XT centrelock ones I had managed to pick up.
It couldn't have been a much worse situation but it was what it was so I set about 'optimising' the set-up. Cable routing and condition was sorted out, pads sanded up nicely and caliper adjusted to perfection. Still not great but better. I left them alone for a few weeks thinking they would bed in, but they actually got worse, to the point of being dangerously ineffective!
Time to get serious! Pads out and they looked dry but a little glazed so these got cleaned with a solvent and then rubbed up again and looked like new. Rotors were removed from the wheels and mechanically sanded until all traces of polishing had gone and both sides had a uniform satin/roughened finish. All now went back together and adjustment once again checked/tweaked to perfection. I then forcibly bedded them in myself and they got better, almost acceptable and now certainly not dangerous, but probably about as good as could be expected for a cheap set of cable operated disc brakes. (I have had no other experience of cable discs but have had the misfortune of Tektro hydraulic brakes which soon went in the bin!)
Over the following weeks, performance fell off slowly until it got to the point where I wasn't happy for my son to use the bike. I was ordering some bits for other bikes so decided to stump up and buy some new pads (
Under £20 for 4 pairs semi-metallic, enough to do the bike twice).
I don't remember doing anything other than popping these in and adjusting clearances but on the first bedding in run the bite went from hmpff. to Whoa! On the second run the brakes were right up at the WOW! level. I would never have believed cable disc brakes could be so powerful, especially cheap ones. I guess the fact that I had done all the groundwork to make sure everything else was optimal helped but these brakes remained really good until we passed the bike on several months later. I would previously have rejected a prospective bike if it had come with cable operated disc brakes but now realise this would be a mistake.
@Crackle I guess this is your problem, the bike your wife experienced probably has contaminated brake pads or they need setting up properly. I suggest you tell the shop to sort it out as they will lose the sale unless they can get the brakes working as they should.