I believe that the cheaper models don't have a transformer built in to them, so I'm guessing if there was a power surge and your charging your iphone, it might get fried.
Hasn't stopped me risking my ipod with a cheap charger! I've used it in UK, France, Australia, New Zealand, Fiji, Thailand, works like you want it to and I haven't encountered any power surges, could be just luck tho 🙂