Nice straw-man. Unity 3D does WORA very, very well. It's been used successfully for cross-platform development by many shipped products. To my initial astonishment, all built on Mono/C#.
This doesn't absolve the developer from QA for the targeted platforms, and this is where fragmentation is the killer IMO. That said, Unity provides a huge amount of leverage and tooling in support of seamless cross-platform deployment.