I think it's important to mention the the performance constraints of different platforms are very different. When I tried porting a Flash game to iOS using AIR to target it, this became apparent as the cost of creating new objects in iOS was huge compared to what it was in the browser, forcing me to use object-pooling, which has a memory cost that could be prohibitive in a different paltform.

This can probably work for simpler games, but I expect the performance and platform-specific bugs to be huge deterrents.