Trying to use [self.extensionContext openURL:... completionHandler:...]; in an iOS 8 Share extension to open the containing app never opens the app, and always calls the completion handler with success = NO.
Here is the same issue with Action extensions, but I think it is more reasonable for Share extensions to be able to open the containing app than Action extensions. The point of a Share extension is to upload a potentially large piece of data, and the only way to do that without opening the app is through NSURLSession, which can only do HTTP(S) uploads. But an app may wish to share content through a different mechanism than HTTP(S) uploads.
Apple documentation doesn't state that openURL... cannot be used for any particular type of extension. It's hard to know whether this is a bug or intended behavior. There is no official information about this.