Ignoring, for a moment, the complexity of deploying software across multiple platforms, it's important to remember the multiple form-factor problem. The same software doesn't work from a user's standpoint on multiple platforms if the UI isn't correct for that device. The best example I can give is "mobile versions" of sites and apps, clearly targeted at the form factor of a phone, being delivered to a 10" tablet. It's crappy.
Bring in the obvious elephant in the room of the number of times this claim has been made from an engineering standpoint and truly failed to deliver, and I yawn every time HaXe is mentioned.