Are non-Apple lightning cables and power transformers safe to use?
Don't these accessories come standard with your iPhone 5 when you buy one?

I was travelling across China a few years ago and I bought a dozen of those 'cheap' adapters. They still charge my iPad and iPhone. So they do work and at first sight, they don't cause any harm to my devices.

But bare in mind Apple's adapters are probably the most safe ones to use. It makes sense a 19$ adapter provide more power, safety and guarantees then a 0,19$ adapter from eBay, DealExtreme,...  There are even people claiming they will harm your iPhone (or your house).

Ken Shirrif has compared like 12 adapters and his findings are written in his blog.

To give a short answer, I have no bad experience in using "fake" adapters (which is no guarantee or reference at all!), but if you want to play safe, just buy the real stuff :)