There are 2 major problems:
1) writing UI code is always tedious, generates a LOT of code, and is very hard to test and verify
2) Any nontrivial APIs are major source of pain in the ass, because they tend to be non-intuitive, poorly documented, or just don't fit the task. I have yet to see an API except for operating systems and programming language standard libraries that is well-documented. Any programmer porting Corona to any platform will be squeezed between two such APIs: Corona and the 3D app.