In the past month, there have been a bunch of posts about BMP and potential problems associated with its use, such as more bone growth than expected including bone growing off nerves.
At my Thursday appointment, I asked the PA about the BMP used in my surgery. Specifically, I asked about what stops the bone growth. She said that in my case, the BMP is enclosed in a "bag" that was placed inside the cage (I don't remember what material the "bag" is made from). Unless something bad happens to the "bag" and it tears, the bone growth will be contained within it.
Based on this information, it will be difficult for me to experience problems reported in other threads.
Knowing this, if your surgeon talks to you about using BMP in your surgery, ask them specifically how the BMP will be used and if it will be completely enclosed in something. If it will be completely contained, there is less to worry about than if it is placed "loosely" at the fusion site without containment.