I have done many many tests with AXUIElementPostKeyboardEvent and I am also seeing the same results as you.
The function call seems to only return viable results when broadcasting to 2 or 3 applications, any more and the function result is unreliable, stating that the key code was broadcast correctly when in actual fact it wasn't. I've spent many hours working on this with Keywire and have finally succumbed to it.
Until Apple actually fix this in the OS any broadcasting software written for the mac will have this problem.
