I transferred ownership of an iOS app that had an watch app included. The transfer went smoothly, however the App ID associated with the watch app: xxxxx watchkitapp did not get transferred to the new owner (neither did the watchkitapp.extension but I could delete this bundle ID myself).
This prevents the new owner from distributing the app due to the conflict with the watchkit app bundle ID not being associated with the new developer. I could not delete the App ID for the watchkit bundle because the app was already on sale via the store.
The bundle ID for the watch target can't be changed in the app, it gets an invalid binary error when trying to distribute a build to the AppStore.
I went through the support process with apple who looked at deleting the ID, however I've been told this can't be done. Also that it can't be transferred by Apple themselves.
So, warning to those who try a transfer for this type of app. You may well end up with an app that is "transferred" however updates cannot be distributed by the new owner.