To your first question, the world consumes over 10 billion wall warts per year, and the failure rate for any of them, let alone legitimate ones, is very close to the mathematical definition of zero. The components are energy-limiting and the plastic is self-extinguishing.
Second question, of course there are clones, both legit and fraudulent. Using the small white cube Apple chargers as an example, legit clones are perfectly safe, but usually have higher output noise, and don't have an Apple logo. Price is a good indicator - *anything* with an Apple logo that sells for under $10 is bogus.
ak