Why is the miller-urey experiment controversial?
The Miller-Urey experiment was basically an experiment that supposedly stimulated the conditions of the early earth, and showed how Amino Acids (the basic molecules of life) could form spontaneously.
This experiment, however, was proven wrong later when it was discovered that they had used the wrong gases, which was why they had received such favorable results. When it was repeated with the correct gases, it did not work. This is why it was shunned, and why I assume it is controversial.
If you want to read more about the experiment, I encourage you to follow these links:
In general though, you should always be ready to expect lots of controversy when you get to the field of origin theory. While it often means easy grades for Biology class, you should be ready for a lot of arguments and controversy if you want to do some formal work on this topic.
Hope that helped :)