They work perfectly well with a little practice
As far as wrecking knives when scraping off the magnesium;
shave it off! don't SCRAPE!
I much prefer a 9-10mm ferrocium rod fire starter, far more reliable and particularly simple in use. The best one in my experience; being the 'Fire Flash' I bought from Woodlore, (quite a long time ago now) with the 'Swedish Army Firesteel', a good second. Most of the blanks I have tried are pretty poor in comparison
'Generally'; best and most easily available tinder: shaved up birch bark, unless you're a townie/city slicker, then use cotton wool balls.
Bow drill fire lighting: savage amusement, good test of cardio-vascular system though and of course it certainly warms you up
The key with ALL fire lighting methods; is PRACTICE.
Even with matches or a lighter, if you don't know how to light the fire, as in; what material to use and how to use it, you will not get a good fire, first time
Practice, Persistence & Patience
cheers
Steve