iOS 7-related accessory issues
In addition to basic physical and electronic compatibility, there are a few issues--as well as one new feature--you may come across that aren't related to the iPhone 5s and 5c's hardware, but rather the fact that they're running iOS 7. (These things affect any device running iOS 7, including the iPhone 5 and recent iPad and iPod touch models.)
External keyboards Keyboards? Yes, keyboards. While most people think of the iPad when it comes to external keyboards, the iPhone supports third-party keyboard accessories. (I use Logitech's Easy-Switch Keyboard at my desk to quickly type messages and other text in several iPhone apps.) And the iPhone 5s and 5c work perfectly well with these these accessories.
However, iOS 7 brings some bad and some good when it comes to using an external keyboard. On the bad side, if your external keyboard has a dedicated Spotlight-search key, that key no longer brings up a Spotlight search under iOS 7. From a gesture standpoint, Apple changed the way you access Spotlight in iOS 7: Instead of pressing the Home button (or swiping to the right) when on the first Home screen, in iOS 7 you swipe down on any Home screen. However, it doesn't appear as though keyboards emulated gestures to access Search--there was some kind of dedicated Search command--so I suspect that this is a bug that will be fixed in an update to iOS 7.
On the good side, iOS 7 has added support for a few new shortcuts when using an external keyboard (thanks to Tao of Mac for the tip). Specifically, when using Safari, several OS X keyboard shortcuts now work on iOS: Command+T opens a new tab, Command+L focuses on (and selects everything in) the location/search bar, and Command+W closes the current tab. iOS 7 is still missing many potentially useful keyboard shortcuts (Command+Tab for switching between apps, anyone?), but these are welcome improvements.
Third-party Lightning-connector accessories The other iOS 7-related issue relates to third-party Lightning-connector accessories. Apple has long recommended that people use only officially licensed accessories--those displaying the Made for iPhone, Made for iPad, or Made for iPod badge--with their iOS devices. However, licensed gear tends to be more expensive than unlicensed, both because Apple charges a fee for licensing and because most licensed gear comes from name brands. So unlicensed gear, which tends to be cheaper, has become quite popular. You'll find it everywhere online, as well as at budget mall kiosks, the shelves of the local discount store...I've even seen it at the gas station down the street.
There are always risks to using knockoff accessories, especially ones involving electricity, but in iOS 7, Apple is being more aggressive in alerting you when you're using unlicensed gear--and sometimes actively blocking that gear from working. For example, I've got a number of unlicensed third-party USB-to-Lightning-connector cables, and after upgrading my iPhone 5 and iPad mini to iOS 7, both displayed an error (shown at right) every time I connected one of those cables. The same thing happens with my new iPhone 5s. A couple of the cables still allow charging and syncing, despite the warning, but two others no longer function at all. (iOS 6 would sometimes display a similar error, but iOS 7 is much stricter in this regard.)
Sign up for CIO Asia eNewsletters.