In Xamarin iOS I get this issue if linking is set to 'Don't Link'
MTOUCH : error MT5214: Native linking failed, undefined symbol: _UAButtonStyleKey. This symbol was referenced by the managed member UrbanAirship.Constants.UAButtonStyleKey. Please verify that all the necessary frameworks have been referenced and native libraries linked.
The same thing happens in the sample app. This isn't a problem in Release mode, but in Debug mode linking needs to be disabled or Xamarin Hot Reload stops working completely. This significantly affects development time on any Xamarin.iOS or Xamarin.Forms project that uses UrbanAirship.