A few weeks ago I got my wife a few cheap lightning cables off eBay but there have been a couple of quirks with them.
First up, they only seem to charge the iPad mini when connected one particular way. Connecting it lightning plug the opposite way results in no change flowing into the device. Perhaps not a big deal when the workaround is to just to twist the plug around the other way.
However, the bigger problem is that connecting an iPad mini to the MacBook Air using one of these cheap cables effectively crashes the computer requiring a hard reset. The screen would just go blank and completely unresponsive to any mouse or keyboard input. The genuine cable creates no such issues.
So, if you’re looking for extra cables just for charging then the cheap ones should still be okay but if you’re wanting to use them for syncing to a computer then you may want to reconsider.