I am going to bust your myths ladies and gents..."
I work on feet all day long as a specialist in this area.
Cracked heels can be painful and can get infected,but they can be eradicated with the help of a foot health professional, you need some bladework to get rid of this and then specialist creams, buying creams and just applying them won't work.
Fungal nails can be eradicated,but the treatment must be applied to the nailbed and this can be painless prepared by a foot specialist, buying nail paint and applying it your nail directly won't work and will be a waste of money. Plus a medical pedicure can be offered by a foot specialist, but with proper equipment so a not to cross contaminate, beauty salons with foot spas are infact the place where most of my patients contract such issues!
And lastly, as hard skin and callus come from friction don't be fooled that your foot rasp is the answer to your problems,because by rubbing you are encouraging further friction and starting the cycle of callus again!
The upshot is, if you need to ,spend some money on getting your feet done properly and then do some good aftercare and your feet will be good to go. Gimmicky rasps,peels etc are just that gimmicks that are not sufficiently in depth to provide any longer term relief.