1. You can't automate running of the UIAutomation tests. Square really wanted this for continuous integration.
2. Writing the UIAutomation scripts is really tedious (though
it's much better in iOS [redacted]).
3. The UIAutomation tests ran very slowly.
4. Having to write all the tests in JavaScript isn't very flexible.
That said, just like UIAutomation, KIF stands on the shoulders of the great accessibility features in iOS. In many ways KIF is like an Objective-C version of UIAutomation.
Our iOS team at Square is more comfortable and efficient
writing tests (and test libraries) in Objective-C than JavaScript. We actually wouldn't gain much from the 'scripting language'-ness of JavaScript, and the additional language and tool (Instruments) would make writing tests more complicated and painful.
(http://developer.apple.com/library/ios/#documentation/Develo...)